Trying to plan backups properly and everything I read says back up your Outlook data file, but my Account Settings shows several files with different extensions and I do not know what each is.
What are these files, which hold real data and which are expendable?
Good question to ask before a backup plan rather than after a loss. Outlook stores mail in two file families and the backup rules differ completely between them.
PST, the Personal Storage Table, is a primary store: whatever lives in a PST lives nowhere else. POP accounts deliver into one, archives are PSTs and any Outlook Data File you created by hand is one. These are the files your backup must cover, and the modern Unicode variety grows to 50GB by default. Find them under File, Account Settings, Data Files, anything with a .pst extension, typically in Documents\Outlook Files.
OST, the Offline Storage Table, is a cache: a local mirror of a mailbox whose master copy lives on a server, Exchange, Microsoft 365 or IMAP. Delete an OST and Outlook rebuilds it by re-downloading, so backing it up is wasted space, with one giant exception: if the server side account dies, the orphaned OST becomes the only copy and suddenly matters enormously. OSTs live in %localappdata%\Microsoft\Outlook and the Data Files tab lists them too.
So your backup plan writes itself: every PST goes into the backup set, refreshed regularly with Outlook closed since open files back up inconsistently. OSTs stay out of it while their accounts live. The Data Files tab is your inventory, each row tells you the type by extension and the location by path.
Inventory done: two PSTs I never knew held my old POP mail plus an archive, all now in the backup set. The OST distinction finally makes sense of every confusing article I read.