"owncloud" is one project that lets you build your own little cloud-fileserver and it can run just fone on the Raspberry. ( example guide https://pimylifeup.com/raspberry-pi-owncloud/
There are different Android owncloud apps but I have no one to recommend.
There is also a Android app called FolderSync ( https://play.google.com/store/apps/deta ... rsync.lite
) which I believe can do just basic SSH/SCP filetransfer also. Then you don't have to do anything on your Pi.