The server, and plausibly at least one secret service. Telegram datacenters are located in Florida, Amsterdam and Singapure.. guess local jurisdiction applies.
Personally I'm pretty happy with GrapheneOS (Android AOSP distribution) and also donating to it. It works for me, its licensed under a free software license, there are plenty of cool free software applications available. I'm familiar with Android application development if I need something custom.
Unless Android (itself, not Google Play Services because I don't depend on them) goes closed source I don't see a reason to switch to something (imo) inferior.
I have nothing PmOS, phosh and other free mobile smartphone projects and will probably redirect my resources towards them if Android goes closed source.
Well the time filters (before/after:date) still seem to work, but for controversial / hot topics, somehow, more recent videos tend to still show up at the top. Try "scandal after:2010 before:2012"..
I've stumbled upon very valuable content with very low view numbers - the algorithms spiral around spectacularity and provocation, not quality or insight.