disable all Yoast SEO options except sitemap

You can do that if you know enough PHP.

Simply edit SEO Yoast plugin files to disable any of the plugin files (or functions) you don't want.

Do you really think that some random bloke on a generic marketing forum asking newbie-level questions is secretly a code wizard?

And even if he/she was it isn't the best idea. You're taking a small part of someone else's plugin which may have all sort of weird structures and dependencies. It may take quite a lot of effort to get an understanding what Joost's team has done, and for what? You're just outputting a bit of XML based on the documents in the DB. All it takes is a XML library, and perhaps a query for every individual document type. Maybe also input boxes for sitemap priority and exclusions if you want to get fancy.

If you're hell-bent to do your own XML plugin for some odd reason you should not reduce an existing plugin, but build your own. At least in name so that you prevent accidental updates. Does that prevent you from spying on Joost's code? Of course not.

Those reasons don't really have anything to do with this. They're Open Source, and PHP. The latter is compiled run-time so the code is usually just sitting on the server.