A few things:
0. Everyone feels like an imposter, especially in the beginning. I'm sure when you were an intern/junior, you felt like an imposter and got over it by continuing to do the thing.
1. Unless your company culture is toxic, I'd also tell your manager that this is your first managing role and you'd love their advice and ask them to fill in your knowledge gaps.
Being vulnerable like that at work might not feel great, but it's way better than needing to remedy something you messed up because you put on a facade that you know everything.
3. It's totally fine to ask your team about things you don't understand. "How did you do this when Ben was the manager? What worked about it? What didn't?"
4. One of the biggest changes from IC to manager is that it's a lot more difficult to fix things with more effort.
Say you're worried about delivering something late. As an engineer, you can work a late night or two and deliver on time. As a manager, it's much harder to fix issues by deploying more of your own time to them.
This is effectively leverage: Things can go very well without much of your input. But when things are going badly, you also can't fix it with just your own time.
Think about it like you used to be rowing a boat and now you're captaining a small crew sailing vessel: There's a crew that can go further, faster, but it requires working well together.