some ereaders do use android, my old onyx did, but honestly i much prefer the dedicated solution kobo has. they could use android, but if they've got the resources to make their own os targeting their actual use case instead of cramming a mobile phone os in there, why wouldn't they? even their os has too much cruft for my taste, but it is a lot less than an android ereader.
No, I forget where exactly it was, but at some point last year I was deep in Rakuten's documentation and it referenced that the Clara HD's OS is based on a modified Android 8 kernel.
Since Kobo rebrands as Tolino too and one i had years ago was clearly some sort of Android, i think they are still? Maybe more locked down and under the hood?