Smartphones, the portability / interoperability thereof

Factual Questions
1

A few years ago, I was not much liking my flip phone and wanted one more like my partner had. Got one very cheaply on ebay that looked similar and went to switch the little SIM chip to it, only to be told by a cellphone-knowledgable person “Oh you can’t do that! Your service is T-Mobile! This is a CDMA phone and your service is GSM!” (or maybe it was the opposite, I don’t know, but the upshot was that they weren’t compatible).

Fast forward to now. My last flip-phone died of battery death and a friend GAVE me my first smartphone, a Samsung Galaxy 4 Note. I moved my SIM card to it and it just worked. Cool! Well, it too is now dying. I’ve replaced the battery but it still won’t hold a charge.

So again I am contemplating buying a used phone on ebay and swapping the chip in.

Do I still need to be concerned with compatibility or is that stuff all ironed out now? People are encouraging me to try an iPhone since I wasn’t all that overjoyed with the Galaxy which is an Android. Can I just buy a used iPhone and pop in the little chip and expect it to work? If not, what parameters govern compatibility with a pay-as-you-go T-Mobile service SIM card?

2

Same problem. There are still different network protocols, and there are different SIM sizes.

3

I think all iPhones and most medium-to-high end Android phones will work on any of the networks. You can probably get a new SIM from the phone company for free if the one you have isn’t the right size.

4

I had an S5 Mini (I like the small size) purchased used and have Straight Talk as my carrier. The phone itself stopped working correctly and I bought another one. They were both marked as unlocked ATT phones on Ebay. I put the old SIM in the new phone and it didn’t work. Straight Talk told me I needed a new SIM (.99) and it worked fine. I recently upgraded to an S10e and needed another SIM card. I think the cards are the same size but didn’t question the reason for the new cards.

5

I recently bought an iPhone 12 Mini, and was told the phone would work better with a new SIM card. So I got a new one from my carrier. It didn’t work, so I stole the old card from my destroyed iPhone 6. It works perfectly.