Learn Finnish: Käyttekö hierojalla usein vai harvoin? - Do you visit the masseur often or rarely?

The verb käydä (to visit) is special. Unlike verbs like mennä (to go) which require a movement case, käydä requires a static location case. Because you are visiting a professional, Finnish uses the adessive case -lla (at), meaning you visit at the masseur's place. This is just like saying käydä lääkärillä (to visit the doctor).

Could I use tai instead of vai for the word or in this sentence?

No, you must use vai here. In Finnish, vai is strictly used in questions where you are asking the listener to choose between mutually exclusive alternatives (either often or rarely). Tai is used in statements, or when the options are not mutually exclusive.

I see the question particle -kö on the verb. Why is the base verb käytte instead of käyt?

Käytte is the second-person plural form (you all) or the formal singular form (formal you). If you were casually asking one person, you would conjugate it in the second-person singular and ask: Käytkö hierojalla usein vai harvoin?

Is the noun hieroja (masseur) built from a verb?