Sorry, I no longer own one. Had one a few years ago tho and was very happy with it. I used to mostly sideload books via calibre through a cable connection but I'd be surprised if it wasn't feasible via WiFi. IIRC I put an android launcher on it so was also able to use the browser to download directly from various free books sites.

I use calibre web for this. It is a bit funky to setup though depending on your level of experience. Here's a guide to look over https://jccpalmer.com/posts/setting-up-kobo-sync-with-calibre-web/

Not natively. As someone else said, you can use calibre. You can also use Grimmory if you have that set up/access to someone's grimmory server, but it does require going into the config files of your Kobo.

Standard operating procedure is just to plug it into the computer and drop files into it.

EDIT: This is said from an owner of a Clara B/W I can't speak to other models.