First time flying seating question

Hi, my friends and I are trying to book an economy flight to japan, we have never traveled before and had a few questions.

Flight that we wanted looks like this: BWI(ANA operated by United) -> ORD(ANA operated by ANA) -> NRT

We looking to book a flight through the ANA website (people said booking directly is better?):

• We(3) wanted to sit together so how do seat selections work?

• I read somewhere that if you book directly on ANA's website, you can choose your seats is this true?

• For both United and ANA, can you choose your seats at the 24 hour only check in or are those randomly assigned?

This might be a dumb question, but since the first flight is operated by United we would check in with the United website online when it opens at the 24 hours, but do we need to check in for the next layover flight operated by ANA?