Which models are they using?

Have a look here: https://www.messenger-matrix.de/messenger-matrix-en.html

I like the comparison and prefer simplex as an replacement to Signal in the Future. Self hosted Matrix Servers are a good idea aswell if you have the money and knowledge.

TIL: Nobody is perfect

I use Eternity btw. https://f-droid.org/packages/eu.toldi.infinityforlemmy/