Basically it is up to you to choose what scenario you want to cover, but it is highly recommended to backup the primary and in many cases to backup the secondary as well
You can check the this document on how to configure backups on secondary replicas of an Always On availability group
Note: In most cases, personally I do not find reason to backup all the nodes if there are more than one replica, but at least one I will highly recommend to back up and in some cases the primary as well for cases I would want to have a fast restore locally. In some cases where the primary is very active you might want to based more on backups of the secondary, but this mean that you depend on the replica to work well and this might be broken so it is important to backup primary as well in many cases. Basically, you can run the backup command from any replica.