Yes.

A pool is just a collection of miners with varying rules governing how they operate.

The pool is effectively just one big miner as seen by the bitcoin network, so sometimes the word "miners" is used to mean pools.

Miners/pools can include whatever transactions they want in a block. Some won't include ANY transactions that don't have a fee, while some will. Some will include very, very few transactions, while others include quite a lot.

The transactions miners include have typically been submitted to the bitcoin network, but they could easily include their own or have a "back channel" agreement with e.g. a payment processor to include their transactions without those ever being submitted to the network (i.e. without being relayed).