RDS Does Not Allow Restoring MS SQL DB from S3
The quick answer is "don't waste your time with AWS and RDS".  But there are nuances.  Those solutions might be appropriate for some things but trying to open a MS SQL backup file on a Mac is not one of them.  Here is what you want to do:

https://apple.stackexchange.com/questions/385430/how-to-read-ms-sql-backup-file-on-mac

The key to it all, for me, was this step by step article that walks you all the way through it:

https://docs.microsoft.com/en-us/sql/linux/tutorial-restore-backup-in-sql-server-container?view=sql-server-ver15