What website/app do you use to read ebooks?

ReadEra.
The free app is so bloody good and adfree already, I bought it just to support the developer after using it for 8 years or so.
Tried several others before and nothing comes even close.
Android, no idea if any other version exists.
- Yes, I use that app too, it's stylish and exactly what I need from an ereader app

Annas-archive.org to get books, ereader prestigio to read them on my phone.
- Thank you for this
  
  I love libraries but their ebook services are crippled by publisher nonsense pretending there are limited copies of a file.
- How do you like Prestigio? Have you tried Moon Reader? Curious to hear what makes Prestigio a good choice (i personally switch between both every now and then)

I use the Android app Librera Reader, since it's the best one I could find that is officially distributed outside of the Play Store (de-google, anyone?).
You can find the APK on both F-Droid and GitHub.
For ebooks, since Librera doesn't seem to be able to validate DRM (thankfully), I buy ebooks in DRM free formats, meaning I usually pend to Itchio, IndieGala, Fanatical, Humble Bundle, Story Bundle, etc.
- Best Android reader!

Lithium on android, I set it so I can scroll down the book rather than flip the pages. Then I get my books on lib gen for free.

Libby is an online library, so you can borrow books from them. They also have audio books. All you need is a library card (which iirc you can sign up through libby).
You can read the books in the libby app itself (not bad, maybe a little clunky) or if you use something like a Kindle, you can have it sent to that, and read like normal. I believe it does others than kindle, but that's what I use, so that's all i know.

- I'll throw a shoutout to Onyx Boox, rather than Kobo or Kindle. It runs Android, can run literally any file type, doesn't lock you into an ecosystem with DRM where you don't really own the books you buy. I despise Kobo and Kindle for that reason.
  But I totally agree about how nice e-ink is. It's literally as good as paper. Maybe better. Soooo nice.
  
  I have the Boox Max, and it's expensive as shit compared to the smaller ereaders, but I use it more than enough to justify. You can't fit a textbook or coding book on a Kindle (probably the scribe you can now), but you can fit two pages comfortably with the 13.3 inch max.
  Android is a huge value add, too. I can't take it for my phone, but for an ereader, it means I can use multiple library apps and scribd to borrow books to supplement my purchases from whatever platforms.

I dedrm my eBooks with calibre and store them on my own NAS. But that’s just for archiving purposes. I read my eBooks on my iPhone with whatever app I bought them, that can be kindle or Apple Books, whatever is more convenient. In the rare case that I actually load one from my archive, or from the Gutenberg archives, or wherever, it’s usually ePub, and therefore Apple Books as a reader.
I also own a kindle device, but find that often more cumbersome, so I use that only in places where I don’t want to take my phone, ie beach or other public spaces where I want to move without much clothing.

On Android I like Moon+ Reader, on my Kobo I installed koreader
- What made you switch to a different app on the Kobo?
  
  I didn't completely switch, koreader is installed side by side with the official software but it has a lot more feature and it suite my needs. Eg. I'm a developer and I wrote an Obsidian plugin to retrieve all my highlights and notes from koreader

Depends on what I'm reading, but I prefer side-loading to my Kobo Libra via Calibre-web. I'm currently taking advantage of a Kindle Unlimited sale and that means I'm reading on an older Android tablet, my phone, or if I'm feeling organized, a Kindle Oasis.
When I do read on my phone, maybe something from Project Gutenberg or Smashwords, it's Librera Reader FD - it's on F-Droid and it's awesome.
If you can, try and find a semi-dedicated reading device as it'll let you focus on what you're reading. The biggest downside to your phone is that you can do other things - like checking Lemmy.

I lost access to one of my Amazon accounts, and it isn't easy to recover. They give you the option to make a paid international call to recover your account, since I don't live in the United States. Can't possibly recommend it. If you lose your account, you lose all the books you bought.
I think kobo identifies your Wi-fi network, but ever since I found out amazon puts you through this, the book buying (actually renting) lost its charm to me.
I would go with whats available freely on the Internet. I use "filetype:pdf" on Google to find books, for example.
- Thanks for the tip!

I use a Kobo Nia it was pretty cheap and I have no complaints at all about it. You can easily copy epubs over yourself with the cable.
On Android I use moon+ but it's easier using the Kobo.

Our public library has the function to borrow e-books (called onleihe, but this counts only for some libraries in germany). I use this a lot.

Another vote for using your library. I mostly use BorrowBox as that's what my library hosts their eBooks on. I have used Libby in the past though.
Bookbub is a site that has low cost ebooks that might be worth checking out.

Try moon reader pro for android, it got project Gutenberg and feedbooks built in and you can configure your own source also it has a lot of necessary feature like darkmode for books and stuff.