As you are new to using Mosaic I would also point out that the Play Queue option has other uses. The obvious one is finding out what track is playing at the moment from an album. If an entire album is selected for play and the Play Queue is opened you will see a blue bar in the left margin that moves against each track as it plays.
Another use is if, say, you have an album of 12 tracks but only want to play it from track 3 onwards. If you are on the home play page you will find that the blue " Play" rectangle at the top is divided into two sections. To the left is Play All but there is a chevron to the right. If you touch this a three item menu appears. Select " Queue". Now open the Play Queue and you will see all of the 12 tracks displayed. Go to track 3 and using the two bar option select “Play from Here”.
I said that the queue is there until deleted. However starting a new session may delete the queue so if you want to keep things indefinitely I would use the buttons to add to a playlist.
Bear in mind though that dCS never intended Mosaic to be an all singing, all dancing application. It is something that is intended to cover basic needs only. I am sure that if pressed they would suggest a subscription to Roon if more sophistication is wanted.