We have a few Kobos (Clara, Libra, maybe a few older ones), but we only use them as e-readers. You “buy” books from the official store at https://www.kobo.com/us/en
In the past they partnered with Walmart and you could buy ebooks there, but that stopped a few months ago and it’s now back to just Rakuten (a Japanese ecommerce company, formerly Buy.com) ownership and the official store.
You don’t really “own” books with Kobo either any more than you would with Amazon Kindle; you just “rent” them for the long term until/unless they decide to revoke your access. But I’ve never had a book removed — thus far.
If you really want your own copy, you have to remove the DRM, which is automatically illegal in the US (doesn’t matter if you paid for it) and which we’re not allowed to discuss on this forum.
That said, you can also find public domain books e.g. on Project Gutenberg or the Internet Archive. And it accepts epubs, mobis, PDFs, etc. out of the box: File formats your Kobo eReader and Kobo Books app support – Rakuten Kobo
Like with the Kindle, you can use Calibre to manage and sync your (self-owned copies) of ebooks: https://calibre-ebook.com/ (no need for this if you only use the official store)
For library ebooks, Kobo has built-in OverDrive sync so you can browse and borrow books directly from the device: OverDrive: Common questions – Rakuten Kobo
Kobo also has its own monthly ebook and audiobook subscription, and the catalog isn’t terrible: https://www.kobo.com/us/en/plus I subscribe to it from time to time, like when I’m going on a trip for a while and want to try a few new titles without committing to them. (But also, you typically get free preview chapters anyway like you do on Kindle)
Beyond books, Kobos are generally more open devices; they’re little Linux computers that you can load other apps and firmware and OSes onto: Quill OS: An open-source OS for Kobo's eReaders | Hacker News
I never bothered with that because the last thing I want is more complexity when I just want to read. But if there’s anything in particular you want to try to do, I can help try to figure it out.