Aside from fees maybe like from token shares. Like trustwallet they airdropped their tokens for free on some referrals. It works like an exchange that when user invites a lot, there are other who will use the app as well and this will increase their user base. Im not sure if there is really an additional fee aside from the on chain transaction being imposed here cause we can see the transaction and confirm in the explorer, I dont see any ads too when using it so it will be coming from trading fees ans transaction fees.