https://skilldrick.github.io/easy6502/
While I have said IRL and on The Amp Hour before that I don’t get why people dabble in old computers, I get it as I get older. Understanding more about what’s under the hood is a good exercise for thinking about compilers and architecture. Also as this site shows: it can be fun
For those of us who got into computers in the 80’s, it’s a nostalgia thing… good old times when the hardware and software world was much simpler 
https://8bitworkshop.com/ is a fancier implementation of a similar concept
My first exposure to assembly programming was on the 6502. It was a great way to start.
I learned 6502 assembly programming at technical college in the UK in 1997.
It was great but outdated by then. I have found that more recent processors are too complex
for assembly and the programming is performed by firmware and software coders over hardware and design engineers.
I very rarely get to write any code these days. I design circuits and perform hardware testing…coding gets sub-contracted out.
I also learned assembly on the 6502. I still think it is a great place for learning assembly. It’s a very simple architecture, easy to start with. That said, I think once the initial concepts are understood, one should quickly progress towards more modern architectures.