You are definitely in the wrong place for that.
TOSEC is a cataloging project.
That said I can give you some hints for what you are looking for.
- There are emulators for virtually everything someone remembers using (and even some for systems that nobody remembers).
- There are "multi" emulators (where a single emulator emulates many machines) but these sometimes (or most of the times) run worse than individual emulators where the developer meticulously improves it.
- Yes you can find TRS-80 emulator.
- Yes you can find Apple emulators although you really need to define what you mean "Apple". Apple is Apple I, Apple II, Newton, Macintosh, new Macs (going to the fourth CPU type change), iPhones etc.
- Google about MAME and about Libretro to get started with things.
- To find "roms" (i.e. old software) is not always totally legal (and most of the times stupidly). There are places to find roms.
- You DEFINITELY need some reading.