Outlook app
seems I may need to download Outlook app for my new iPhone 13 - which app do I choose = Outlook or 365 (we have 365 on the PC)
iPhone 13, iOS 15
Outlook is the name of the mail client no matter whether it's associated with the 365 subscription, or the 2019/2021 perpetual license version.
Since you already have a 365 subscription, it's just a matter of which one you have:
If the single user license, then no, you can't install it since you won't be able to activate Outlook on your iPhone unless you first deactivate 365 on your PC.
If you have the standard $100 per year version, that includes six licenses. Meaning, you can install and activate Office on any six devices you want - iOS, Mac or Windows.