Page "app" vs Subdomain "app"?

Important question: “app” subdomain or “app” page?

I’m thinking about the best architecture for my site. The first product is an app, so a single page on an “app” page (all features on this page, mobile app iOS and Android). Then I have a few other pages “get involved” or “about” and “legal”. Quite classic. But what about having a subdomain “app.domainname” instead of “domainname/app”? What are the advantages/disadvantages with Bubble? In the future I might separate for example a complex “database” page with various filters, an “app” page, a “research” page, as really different offers/services.

So it might make sense to have sub-domains instead! What are the speed/SEO/server implications?