Generally no. Big tech companies have gotten good at locking down devices to the boot loader. Some of the signing keys for certain OTA versions have leaked, but you can’t rely on that.

Some of the devices contain browsers, and people have set up hacky ways to turn them into thin clients through that, but it’s not particularly reliable IME.

I heard some Chinese brands which made similar hardware for Chinese consumers don’t lock their devices down, letting you flash an open install of Android on them, but I haven’t seen anyone try that IRL.

Yes

https://xdaforums.com/t/unlock-root-twrp-unbrick-amazon-echo...

Youtube is trying to push me to watch a video about jail breaking the Echo Show for a week now. I didn't watch it, but it's probably easy to find.