I was avidly reading IntelSlav Z etc. at Telegram and my phone available memory gradually disappeared until my phone wouldn't function.
It turns out Telegram defaults to storing media on your phone. It is very easy to fix. Instructions are here:
https://www.guidingtech.com/top-ways-to-free-up-space-in-telegram/
The information I get from Telegram on Ukraine is outstanding and available nowhere else. Telegram was actually founded by a Russian.
Based on what Telegram actually does I really doubt it is FBI.
Just turn off the right buttons (see instructions) and it stops the saving of files.