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.