Question about Making NES Homebrew

[C4_KadalMen]

Is it possible to make NES Games with just Assembler on MS-DOS?
I saw a lot of NES Bootlegs and I wonder how they create it in the past.

If possible, where should I learn Assembly Language for NES?

[FAST6191]

Sure. Might be a bit limiting in some ways (nice to have a level editor, game text editor, graphics programs you can use a mouse with https://www.youtube.com/watch?v=aMcJ1Jvtef0 , music composers, source code revisions and emulators with a nice graphical interface, possibly all running at once) but there are plenty of amazing coders that quite deliberately opt for "might as well be DOS" looking text editors to do their work in over a modern IDE approach, indeed often shunning IDEs as having downsides (and it can make for lazy code).

I am not sure what we are pointing people at to learn 6502 NES assembly and hardware. Can't really go wrong with https://wiki.nesdev.com/w/index.php?title=Nesdev_Wiki though.
For learning assembly you might find an older book on it (indeed I have such a book on the bookshelf next to me) but they might not do the best teaching style. To that end I often point people at X86 assembly and say come back after you have the fundamentals there https://www.plantation-productions.com/Webster/ (see art of assembly) and Gavin's guide to x86 assembly https://stuff.pypt.lt/ggt80x86a/asm1.htm being my usual links there.

Most around here though when meeting assembly care more about hacking games, and that usually means starting small to change some existing code in a meaningful way, not having to worry about all that one seeking to have a whole running program needs. For that things get a bit gentler and you can introduce people to the three major classes of assembly instructions, or maybe do the everything is adding thing and go from first principles electronics (I come from that background more than straight coding, worked for me and several others where more abstract stuff fell short).

[Jorpho]

Seems X816 was once wildly popular for assembly. (Someone was still trying to use it just a little while ago, as if no better replacement had arisen since the DOS era.)

[Bregalad]

Is it possible to make NES Games with just Assembler on MS-DOS?

Yes, although there's no reason to limit yourself to MS-DOS. If you want nice graphics, a tile editor will be a must, because typing numbers manually will quickly become infeasible.

I saw a lot of NES Bootlegs and I wonder how they create it in the past.

Homebrew or bootleg ? This is not quite the same thing.

If possible, where should I learn Assembly Language for NES?

You should look for 6502 assembly tutorials. The book by Rodney Zacks is also great for learning 6502 assembly. The NES is thankfully by far not the only machine with this lovely little microprocessor.