Sending, receiving, and organizing email in Outlook.com
There is a difference between Outlook email service and the Outlook 365 app.
Outlook email service is a feature of a Microsoft account. So is Microsoft 365. You can use your Outlook email service with the Outlook 365 app, if you want to, and most people do. But it doesn't have to be that way.
You can, if you want, use a different email service with the Outlook 365 app. You could use the Ourlook 365 app for your Yahoo Mail or your GMail or your Proton mail, to give some examples. But you'll still have your Ourlook email service.
You can use your Ourlook email service online, or your can use it with a different app, like GMail.
Just keep in mind the different between an email service and the app that you use to receive that service. Also keep in mind that anyone who offers an email service will let you use their service online, without any app at all. I use my Outlook and GMail service totally online, without any app.