Flash messages: Where are they stored and how can I manage them?

Flash messages appear on all my devices {iPhone, iPad and Mac desktop} only briefly and then spontaneously close. Some on Mac Desktop remain until deliberately closed by me. This is the ideal situation.

As for the hundreds of other flash messages that briefly appear and then close, how can I go somewhere and actually read them. Of 100 flash messages, 10 will end up in Apple messages, 2 in Messenger messages and he rest somewhere in the ether.

If I cannot get to the rest and read them at a time that suits me {I don’t sit glued to my iPhone screen 24/7} then they are totally useless to me. In that case, how can I turn them off because I would call them unsolicited correspondence.

If I can get to the rest, where do I find them and how can I manage them?