But back to the mechanics of it.
You can have a Bitcoin and spend some portion of it in a transaction. So you have a Bitcoin with less value in the block-chain accounting. It is divisible to 100 million parts. I still have no answer as to how you can get plus Bitcoin. A Bitcoin is mined. It is a separate entity. Can your Bitcoin gain 100 million parts? Or more?
How does that work? I am a seller of goods. I have one Bitcoin. How do you give me more Bitcoin for my goods?
You would have a “public key” that a customer can use to deposit Bitcoin to your wallet.
It’s like talking to a brick wall.
I have a physical Bitcoin token purchased from Amazon that blows peoples minds. 
On spending a Bitcoin to zero.
So I have one fraction of a Bitcoin left. And I spend it. That is 1, 100 millionth of a bitcoin going to a seller of the goods that I bought. They get One 100 millionth of a Bit coin. I now have zero Bitcoin.
What happened to my Bitcoin? It was a mined, individual Bitcoin. I finally spent that last 100 millionth of it. Does it cease to exist? The person I bought from received the last 100 millionth of my Bitcoin. Now what? Is that Bitcoin gone?
I am a seller of goods. I have one dollar. How do you give me more dollar for my goods?
I have 1/100th of a dollar. I finally spend that last 1/100th of a dollar. Does it cease to exist? The person I bought from received that 1/100th of a dollar. Now what? Is that dollar gone?
Some circular logic in there. But ultimately still reinforcing my feeling it is a bank account.
How does a Bitcoin that is spent to zero magically change hands? I bought a digitally mined entity. A commodity. A marker in a dispersed accounting scheme. That did not become yours because you were the last seller that I spent my last portion of it on. You would not get my bank account, because I spent my last dollar in to buy from you.
DOH!
There’s no magic involved. If you have spent your bitcoin, it has changed hands; that’s how you spend it.
If you spent it buying somethig from me, it does become mine, just as the something becomes yours. That’s how spending works.
But I would get your last dollar. Similarly if you spend your last bitcoin, or your last fraction of a bitcoin, in buying something from me, you no longer have that bitcoin/fraction of a bitcoin; I have it. You spent it; you gave it to me; now it’s mine.
You still have a bitcoin wallet; it’s just that there aren’t any bitcoins in it. You spent them.
And it’s wrong to say that you “spent your bitcoin to zero”. You spent your bitcoin, but that didn’t affect the value of the bitcoin, just its ownership. You have a zero balance in your bitcoin wallet, but the bitcoins that you took out of the wallet and put in the wallets of other people still have a value of one bitcoin each.
If I spent my last 100 millionth of my bitcoin to purchase from you. You would not get a Bitcoin. Just 1, 100 millionth of one. The Bitcoin is a mined entity. It is unique.
I am not getting any clear answers. You would get the last portion of the value contained in my Bitcoin. But you would not get my Bitcoin.
Let’s not forget to mention the fact that spending Bitcoin here and there for a pair of shoes or dinner or whatever does in fact come out of your wallet, but will, because of the growth in value of Bitcoin re-appear as dividends in your wallet.
I have a certain amount in my wallet as a “seed” and reap the benefit of dividends leaving my “seed”.
Maybe it would help to replace Bitcoin with a different ‘mined entity’.
I have 1 bar of gold that I mined. I want to buy things with my gold but the value of the bar is too great for me to spend it in one go.
So I remove a small piece from the bar and give that to a merchant in exchange for goods. I now have a slightly smaller bar of gold. This process continues until I’m down to my last gram of gold.
I then buy some small trinket with my gram and hand it over. I now have no gold left. I started with 1 bar and now have 0 bars. I cannot say that I still have 1 bar of gold that is worth 0.
Once you spend your Bitcoin, in pieces or in one go, it is gone. You now have 0 Bitcoin.
Bitcoins are not like bank accounts. They’re like dollars. You can spend bitcoin from an account in the same way you can spend dollars from an account.
That’s correct; I just get part of your bitcoin. The rest of goes, or has already gone, to other people.
You may have mined your bitcoin, but that does not make it unique, any more than a gold ingot is unique. Your bitcoin is exactly like every other bitcoin. It can be divided into parts, like a gold ingot can, and the parts can be given to different people, who will add them to the bitcoins, or to the gold, that they already have.
The whole point about money is that’s it’s fungible. One dollar is absolutely interchangeable in all circumstances with another. If I lend you a dollar by handing you a dollar bill, when repayment day comes I am entitled to demand that you pay me a dollar. I am not entitled to demand that you return the particular dollar bill that I gave you. Same goes for gold ingots. Same goes for bitcoins.
It is an interesting question though - what happens to fractional bitcoins in your wallet? Are they aggregated into a single bitcoin of larger value, or are they kept separate?
It might help to realize that the bitcoin isn’t actually the fundamental unit. The actual fundamental unit is one hundred millionth of a bitcoin. They’re only bundled into big bitcoins because the fundamental unit is too small to be convenient. It’s like dollars and cents.
You can have a hundred pennies in your pocket. You buy things with them, and the number of pennies you have decreases. You start with a dollar in a pocket, but after you make your first purchase, you don’t have a dollar in a pocket (not even a dollar worth 97 cents); you have 97 cents in a pocket. After you’ve spent all of it, you no longer have any dollars or cents at all, but you still have a pocket which you could in principle put money back into.
You can have a hundred million bitcoin-pieces in a wallet. You buy things with them, and the number of bitcoin-pieces you have decreases. You start with a bitcoin in a wallet, but after you make your first purchase, you don’t have a bitcoin in a wallet (not even a bitcoin worth 97,000,000 bitcoin-pieces); you have 97,000,000 bitcoin-pieces in a wallet. After you’ve spent all of it, you no longer have any bitcoins or bitcoin-pieces at all, but you still have a wallet which you could in principle put money back into.
This thread is bizarre. It’s exactly the same as “if I buy something for a dollar, and I give the vendor my dollar, where did my dollar go?” You have one less dollar, he has one more. Same as if you transfer bitcoins…
The OP seems to believe there is some privileged status associated with “one bitcoin”. There is nothing special about having “one bitcoin”. Absolutely nothing. You don’t and can’t mine “one bitcoin”. New bitcoins are currently generated 12.5 bitcoins at a time, when a block of transactions is successfully validated. Some time in 2021 it will halve (again) to 6.25 bitcoins per block, and when all 21 million bitcoins are generated, the block reward will become zero.
“Mining” is an unfortunate misnomer. It’s really the process of validating Bitcoin transactions. All miners compete to find the mathematical solution that validates the current block of transactions. The entity that finds the solution receives the block reward. They can then go on to do what they wish with the block reward, such as sending to any other Bitcoin address in any combination of amounts they want.
It might be possible the OP heard somewhere that a bitcoin transaction requires all of the balance from one address to be spent in a single transaction. This is true, but is a technical detail that has nothing to do with single bitcoin amounts. It works like this: