No. Microsoft 365 (previously office 365) is not a web app. They have web apps, and some licenses (the bare bones $6/mo one) only has web apps. But overall the suite of apps can’t be defined as web based.

Not to be confusing, but some of the apps are only web apps, but those are “other” apps than you’re probably thinking of. Like Planner or Power Automate. The “office” apps like outlook, word, excel and PowerPoint all have desktop and web versions included.