If I sync my Joplin notes to my local file system (Android) how could I transfer that to another platform?
I use SyncThing
I have set up a free Nextcloud account (5Gb). Joplin syncs to that, from my Android phone, Macbook (work) and Linux (home). Works great. For the MBP and Linux the Nextcloud desktop client is necessary. I bet it works likewise in Windows.
It depends on which other platform you want to use.
I think that rsync is the easiest solution if you do not have Webdav folder somewhere.
On Windows it is called Acrosync https://acrosync.com/home.html
on windows you can use the gui ...
and on a Linux or MacOS terminal the rsync syntax is very simple :
rsync -rtuv <source> <destination>