Managing engineers is a new career, that is separate from being an Engineer. Many engineering skills don't transfer to management, even when you think they do.

As a manager, one of the most important things you can do is schedule regular 1 on 1's with the people who report to you. Both "The Manager's Path"[1] and "Behind Closed Doors"[2] stresses this.

In about 4 months, it'll be helpful to review PG's essay, Maker's Schedule, Manager's Schedule[3] Right now, you'll be coding most of your time, but you'll soon have more and more meetings. MSMS names the feeling of frustration around meetings, and describes how to handle so many meetings.

[1] https://www.amazon.com/Managers-Path-Leaders-Navigating-Grow...

[2] https://www.amazon.com/Behind-Closed-Doors-Management-Progra...

[3] http://www.paulgraham.com/makersschedule.html

You say "the Senior", so maybe this also means being in charge of other people? If it's just individual contributor, comment from emeraldd is good advice.

If you'll also be a team lead, https://www.amazon.com/Behind-Closed-Doors-Management-Progra... is good book to read on management.

Also: spend the first 30 days just listening as much possible, not making suggestions. If there are things that need improving go and try to improve them directly yourself, or do it afterwards. But if there are problems make sure you spend the time to figure out why people are doing bad things, and don't just criticize.