The TL;DR is that you are now responsible for anything that is not moving your team forward. If nobody knows what "forward" means, it's your responsibility to define it. If a code review is languishing, that's your responsibility. If a person has a technical question and they're junior enough that they don't know they can ask it, that's your responsibility. If your team doesn't have a good technical strategy, that's your responsibility. If your team is having trouble getting technical help from another team, that's your responsibility.
It's not your job to do all of these things, but it's your job to make sure they get done.
https://www.bitlog.com/2017/10/12/what-does-a-tech-lead-do/
The TL;DR is that you are now responsible for anything that is not moving your team forward. If nobody knows what "forward" means, it's your responsibility to define it. If a code review is languishing, that's your responsibility. If a person has a technical question and they're junior enough that they don't know they can ask it, that's your responsibility. If your team doesn't have a good technical strategy, that's your responsibility. If your team is having trouble getting technical help from another team, that's your responsibility.
It's not your job to do all of these things, but it's your job to make sure they get done.