A feature phone is somewhere in between a true "dumb phone" and just a basic smartphone. In practice there's a grey area, but feature phones can do some basic basic stuff with 2G and 3G data. Feature phones vary, and sometimes only have a carrier or manufacturer app store, so sideloading Signal and just not accepting video posts would be possible on a phone from 2005 or so.

But since the feature phone market is very niche in developed countries, it's not the same as just all the phones running on one of two operating systems.

To answer the question of how does one install Signal on a feature phone? It will really vary by phone. Likely just moving a JSON file or something onto a the phone, but just a guess.