[catlikesshrimp]

It is a solved problem https://f-droid.org/packages/com.ismartcoding.plain/

There is also localsend, but plainapp needs to run in only one device.

[extraduder_ire]

I've been using pairdrop.net (fork of snapdrop, which got bought out) a lot recently. Only needs a web browser on either end, and doesn't take any prepwork.

Can be self-hosted, if that's a concern.