The basics: how various (Microsoft account) websites fit together

Let me add to the confusion 🙂

Because you have a "business" account, it's technically not *yours*, but belongs to your company/organization, as in the entity you configured when initially creating the tenant. So while you can access your info and even configure some settings in the myaccount portal, there can be situations where certain things are handled only by the organization's IT stuff. And the admin portal (2) is the primary interface for performing many of these tasks. Primary, but far from being the only one, in fact there are a dozen or so "admin" portals as part of the service currently. The Azure portal (4) being one of them, specifically the Azure AD is the "directory" where all user's data is stored. For a one-man shop, you really shouldnt care much about those though.

The Office landing page (3) is where the end user will usually end up (or start from), and it's intended to offer a quick overview of all his apps and data stored within the service. How successful it is in this regard is up for debate.

And yes, there's a lot of confusion that stems from bad naming conventions used by the marketing folks. Office 365 can mean both the set of desktop apps (Word, Excel, etc) and the cloud service that hosts your email, stores your documents and so on. Nowadays they've rebranded the former to "Microsoft Apps", which is hardly an improvement if you ask me. And there's a lot of overlap with other Microsoft services, such as Azure, as you've already noted.

Again, for an end user/one-man shop, you shouldnt be concerned with those too much. Use the desktop apps as you've used to do in the past, take a stroll through the office.com landing page to explore the other parts of the service, and if you want to learn more, watch some of the video materials Microsoft has posted online.