They have paid tiers. Yes you can get basic email for free, but it has a limited inbox size and filtering options. I think their cheapest is $4/month (USD) which also includes some of their other products. They have higher tiers which expand storage even further, allow for custom domains, and provide access to beta features.

Proton also has a VPN, encrypted storage space, and password manager. They're slowly building a full suite of privacy-focused apps with paid tiers for all of them. Development stagnated for a while but it seems like they've made decent progress recently, although their products still don't match the features of Google's app suite.

Full disclosure, I have been paying for Proton services for several years now and use them as my primary email provider.

We have a freemium subscription model. Users upgrade to get additional storage, more custom domains, priority support, etc. Because our users pay us to protect their privacy, our financial incentives and interests are perfectly aligned with theirs.

The free Proton is very limited, so I do guess most real users pay Proton.