Electrum Server running. Now what?
Here is the protocol: https://electrumx.readthedocs.io/en/latest/protocol.html. If get_mempool serves your use-case then you may use it. You may need to experiment with it to find what is more convenient with Core.
Some apps might be using the Electrum protocol under the hood. Only Electrum allows setting the server manually.
Electrum protocol has blockchain.scripthash.get_mempool which doesn't help you build a mempool observer. You need to connect to Core directly then