Looking for advice on how to handle backups

I am a little unclear on how some of the backup functions in SQL Server 2014 work. First, let me describe my goal.

We have a Dynamics CRM 2016 On Premises server and a SharePoint 2013 On Premises server.  Our SQL Server provides database services for both.  I want to create weekly full database backups of all databases associated with both products, and run transaction log backups hourly.  In case of major failure, our longest possible period of data loss is one hour (and likely to be no loss at all if we can recover the tail logs).

When creating an SQL server backup, there is the option to append a backup to an existing backup file.  There is also the option to define how long backups are good for before they expire.

I am interested in the expiration function because I don't want to keep backups indefinitely.  The storage requirements could get cost prohibitive.  Also, bloating the msdb database with backup history can harm performance.

I am unclear on the interaction between an expired backup within a particular backup file.  I am trying to decide if I should keep all full database backups in a single file per database and all transaction log backups in a single file per database.

Here's my question: if I keep all my database backups in a single file, and all my transaction log backups in their own file for a particular database, and I set the expiration of a backup for 3 months, will SQL Server mark the space inside the backup file occupied by a backup greater than 3 months old as free space, and overwrite it as needed?  If that's the case, then I would only need a background task to run to remove backup history for expired backups from msdb.

Second question: If every backup goes into its own backup file, timestamped, will point in time recovery still work just like it would if I relied on a backup file with multiple backups in it?

Third question: this is a terminology question.  When SQL Server refers to a "backup set", is that a single backup at a particular point in time, or is it a backup file with multiple backups present in it?