Basically this would be used mostly for micropayments, and users inside the app can send crypto to each other without fees, only during withdrawals outside of the platform, some fees would be applied.
So, you need to manage central databases. inside updating of those database (send money to others inside website) is not related to Bitcoin, and the role of Bitcoin is limited to confirming the transaction, and update main balance.
You do not need altcoin, but all you need is to use the lightning network, as it will be close to zero.
You can pool all of these transactions to a single address in cold storage or from a Lightning Network channel.
To withdraw, if your site is small, it is better to do it manually, including reducing risks.
The best open source solution is
https://docs.btcpayserver.org