Content strategy, keyword research, Search Console integration, site speed improvements, local search business profiles -- all of these contribute to SEO. It's unwieldy to talk about them all, so sometimes people can use a shorthand.
I'd always give the same advice. Don't try to outsmart the search engine, focus on quality and consistency over time. That will always reward you in the long run. Try to game the search engine and you will find yourself in a weapons race you can't sustain over time. If you find exploitable tactics to get better ranking than you deserve, sooner or later you'll probably be pushed down the ranking. And this is really important if you're successful enough to have something to lose.
Some of what people label SEO is just "make good web sites", and that indeed works. But some of it is just pure rubbish. And making a good website isn't just "doing SEO" - it is making a good website. And a lot of people don't.
Which is why I can't take people seriously when they present it as some singular task they have completed.
You would have these very complete theories of how search engine ranking worked - completely disconnected from reality and unburdened by any actual knowledge. If there was ever a cargo cult, SEO was it.
I still cringe when people say that they have "done SEO" on their site while I'm in the room. It is kind of like telling their mechanic they've sacrificed a goat so their car will get better mileage.