There's a lot of things that happen in the hood of a simple looking product, take for example a fairly okay sized booking platform for hotels, it can have multiple products like sponsored placement for hotels, customer review and rating services, listing management and verification services, payment services, internal services like customer service portal, and other internal features like recommendation systems, international law specific features etc. Now think about how Booking.com and other sites actually offer more than this, they have flights, buses and so on different verticals. When you consider these and then think of something like Amazon or Microsoft then you get many features to work on even for an individual PM.