as far as I understand, both these systems are proprietary, and it is not clear to me how to get access to it to learn to program it and, yes, then make money from customization work 
Anyway, so can anybody tell me at least what are the mechanics of the programming for these systems? Is SQL used? What is the language like? What kind of customizations are “typical” and common, sort of what is the usual “hello world” type of work there? Are there open source scripts or programs or whatever that could be installed and played with if I were to get access to an installation/account in the system?
SAP is a Enterprise Resource Planning (ERP) system that is massive and is composed of different modules. It basically runs medium sized to massive corporations. There is no one answer to this question because it is so big.
You need to know corporate databases and SQL like the back of your hand but also business analysis, business systems analysis, and systems analysis. Those are good skills to have no matter what the software used is. SAP consultants usually make a lot of money because they understand both the business side and the software side to make things work.
We have at least one SAP consultant on this board and I work some in SAP but also many of ERP software solutions. It is lucrative but it isn’t for the faint of heart. You have to understand how technology intersects with business needs to get a job in it. There are SAP training classes that you can pay for that would help but that software is massive and everyone specializes. Project management skills are also good ones to have. A good SAP person makes in the six figures but builds up to it over time and usually has to travel and move a lot although a lower level SAP person could do some of it remotely.
I have never heard of SalesForce before but it I can say it isn’t in the same demand league as SAP.
SalesForce is a hosted solution (not sure if it can be run locally or not) so this one might be difficult to do much with.
Edit: I just googled, looks like they have a language for customizations.
SAP is programmed in a proprietary language called ABAP (do you know German? then you can read the error messages). As the Shagster said, it’s big and you need to learn a lot before you can properly modify, but people do it and make money.