To give you a better picture: Transactions aren't being picked up and delete it from the mempool. (This must be where your confusion came from)
Mining Pools, Solo Miners, anything with Full Node (eg. Bitcoin Core) have their individual mempool and it's constantly being updated based from their own/default rules and new transactions from their peers' mempool;
it will only be deleted or rejected if it was "mined", didn't follow their set or default rules, input(s) were double-spent or transaction expired.
So each node's mempool shouldn't be too different than each other's.