Whenever I enter crucible matches or other online modes, chat says 'Type /t to join chat' but couldn't find anywhere on google what it means. How to join text chat?
There is no public chat in Destiny 2.
If you enter a Crucible you are initially placed in the fireteam chat, you can switch to the Teamchat with the /t command. The Teamchat theoretically only makes sense in the Crucible since you are not in the team with anyone outside of the Crucible activities. To switch back to fireteam chat you have to type /f (correct me if I'm wrong).
At the time of posting, you are only able to chat with your fireteam or your team in the Crucible. Because of this, many players say that this is one of the most unsocial MMO's out there.
TIP: You can always check which text channel you are in by writing something in the chat (no one is going to see it anyways).