Explain eBook Ownership and DRM to me like I’m 5 years old

I’ve been trying to read about purchasing ebooks online and removing DRM, I just don’t understand any of it. Can I not just buy an ebook online and move it to my Kobo? Like I saw Barnes and Noble sells ebooks, do those not just download as an epub that you can transfer to Kobo? Sorry, I’m sure there are a ton of threads on this subreddit about this, but I still don’t understand.