It’s not trivial for Kindle anymore. I did it recently on a Mac, and it took a fair bit of work, was complicated and confusing, and required finding an old version of the Kindle reader program and somehow preventing it from checking in with Amazon’s servers. My brother in law recently tried and failed after having done it in the past.

It's still trivial AFAIK (I last did it one or two years ago) if you have an old kindle device, and can therefore convince Amazon to send you the poorly encrypted files meant for old kindles. I use a "Kindle Keyboard" e.g. the 3rd gen. Amazon has stopped selling new-old-stock, but it seems you can still find them on ebay for somewhere between $20-50.

Incidentally this model of Kindle also comes with text-to-speach, which is a really nice feature which I believe is missing from all newer models (removed to avoid cannibalizing audio book sales.)

Keep in mind that by buying DRMed books you give companies an incentive to produce them. If you're opposed to DRM, it may be better to pirate instead. Especially if you live in a jurisdiction in which removing DRM is illegal so you're breaking the law anyway.