1st time traveling since the seating assignment change

We had 3 travelers total: me, my wife (who is my designated companion for companion pass), and my wife’s friend.

We got basic tickets so we didn’t pick seats before hand and just let things play out. I didn’t check in right at 24 hours before the flight since I figured it probably didn’t matter. Since I booked all of the flights on my account, I and my wife’s friend had the same confirmation number and I had to check my wife in separately as a companion.

They seated my wife and me 1 row apart (17 and 18) and we were able to trade with our neighbors to sit next to each other.

They seated my wife’s friend in the back row 30

On the trip home, they actually seated all 3 of us in the same row 16 DEF. I checked everyone in when I remembered within 24 hours of the flight.

Both flights were full

Is all of this randomly assigned? Or does check in time actually matter?