It very much depends on the kind of bridging you want to do and on the service you want to do it with. My iMessage bridge, for example, is a "double puppeted bridge", meaning the people receiving messages from me via iMessage don't even know I'm using a different service, and they show up in Matrix as if I got a message from a matrix user. It's also possible to bridge the contents of an entire room (on, say, Slack or Discord) into a matrix room. The two rooms mirror each other and anyone who talks in either room has their messages relayed to the other.
A good place to get a basic rundown would be here: https://matrix.org/docs/projects/bridges
For the puppet bridges (which is what I'm running), you can take a look at this repo: https://github.com/matrix-hacks/matrix-puppet-bridge. You're also very welcome to ask around in #matrix-puppet-bridge:matrix.org about getting the bridges set up. We're pretty friendly in general :)
A good place to get a basic rundown would be here: https://matrix.org/docs/projects/bridges For the puppet bridges (which is what I'm running), you can take a look at this repo: https://github.com/matrix-hacks/matrix-puppet-bridge. You're also very welcome to ask around in #matrix-puppet-bridge:matrix.org about getting the bridges set up. We're pretty friendly in general :)