There’s a lot of answers itt but heres a simpler one:

If you want to prevent people in power from having access to communications there are two methods employed, broadly speaking:

The first is to make a very secure, zero knowledge, zero trust, zero log system so that when the authorities come calling you can show them your empty hands and smirk.

Signal doesn’t actually do this, but they’re closer to this model than the second one I’m about to describe. Bear in mind they’re a us company so when the us authorities come to their door or authorities from some nation the us has a treaty with come to their door signal is legally required to comply and provide all the information they have.

The second is to simply not talk to the authorities. Telegram was closer to this model than signal, using a bunch of different servers in nations with wildly different extradition and information sharing mechanisms in order to make forcing them to comply with some order Byzantine to the point of not being worth it.

Eventually the powers that be got their shit together and put hands on telegrams owner so now they’re complying with all lawful orders and a comparison of the tech is how you’d pick one.

The technology behind the two doesn’t matter really but default telegram is less “secure” than default imessage (I was talking with someone about it so it’s on the old noggin’).