Arc Browser does not have a public monetization plan. The Browser Company is a for-profit company, and they'll need to give something to their investors. Their market share now is abysmal to make a meaningful amount of profit from just selling user data.

At some point, they will need to introduce some kind of subscription for some of their browser features - I assume that current Arc Max is going to be part of it. This would lead to making new features available only to the paying users.

I have really pessimistic views on software development (for-profit or not) in general now, so I'd be really happy to be wrong here.