I think most PWAs are essentially front-end clients to a SaaS, so revenue typically comes from user subscriptions. Other than that like another user mentioned

I think most PWAs are essentially front-end clients to a SaaS, so revenue typically comes from user subscriptions. Other than that like another user mentioned there's any monetization methods on the web at your disposal (advertising, microtransactions, donations etc) it really comes down to what business model you want to operate on and what seems most natural for users of your product. As far as a "buy it, you own the copy" model like with traditional media software and games go I don't think there is anything universal to use, most developers would implement their own licensing in their billing/accounts layer and expose a serial key or something that local data gets encrypted with as a rudimentary DRM, so the app can only be used later by having the right serial key (whether from LocalStorage/cache/a fresh login) for that paid user to decrypt and use their local data. It would be interesting if some service like PWA Store were to offer some kind of universal DRM to developers wanting to sell offline PWAs on a one-off sale format that they can easily wrap their app bundle in