On the Android ereaders that we can install apps thereon, we can install some app (reader, file manager etc.) with wi-fi transfer capability and transfer ebooks between our phone/tablet and ereader over an existing network (using web-browser on our phone).
On quite a few ereaders we should be able to simply use its web-browser and transfer the files over wi-fi using any app (reader, file manager etc.) with wi-fi transfer capability installed on our phone/tablet.
If we want to be independent from any wi-fi network (at home, caffe bars etc.) we can use some cheaper wi-fi memory stick, dongle, HDD enclosure etc. that will create its own wi-fi network.
https://www.colourmylearning.com/201...-flash-drives/
If our wi-fi memory stick, dongle, HDD enclosure etc. has also got a host-usb connector we can then attach any older or newer ereader to it using usb cable and then wirelessly transfer ebooks from phone/tablet into ereader (using dedicated android/ios app), as I would e.g. transfer ebooks wirelessly to my Kindle DX plugged into my wi-fi-HDD-enclosure using usb cable, even though Kindle DX has got no wi-fi capability (just 3G, now rendered useless).
On some Android ereaders we should be able to transfer ebooks wirelessly even without an existing wi-fi network, as we would transfer the files between android/ios phones/tablets, by using cross-platform apps for wireless file sharing between iOS/Android/PC/Mac devices, like AirDroid, SHAREit, Zapya, Xender etc. because some of them don't even need existing wifi network (wireless access point) 'cause they create their own direct wifi connection (Airdrop app) i.e. something similar to using IR (Infrared transfer) or Bluetooth file transfer between two paired devices but using wi-fi instead.
https://techwiser.com/best-airdrop-alternatives/
Also, as already mentioned we can transfer ebooks wirelessly between devices using Koreader app on the same network (Zsync transport plugin)
https://github.com/koreader/koreader...sync-transport