Officially it's "business casual" but I wore a pair of dress slacks and shirt. In other words, a suit sans tie and blazer. They are pretty casual, but you never want to give a bad impression.

The onboarding mostly involves a tour of the facilities and then sitting at a table with an HR person and pilot and answering a few questions. The logbook review will be trivial if you already have an ATP or more involved if you don't. Make sure you have tabbed all checkrides in the logbook for easy reference and provide a sheet of totals for all of your times. Don't make them work to figure out whether you qualify.

Just make sure you bring all of the things they ask you to bring, including all of the forms filled out to the extent possible, as well as IDs (including passport).

It's a relatively simple process. By this point you've passed the interview and they're just doing paperwork. Relax, smile, be prepared to ask a few questions, and enjoy. Not much more to it.