Please explain how gas prices work (where one buys gas) for sending transactions.

I'm now to ETH. I have trouble finding answer via googling. Example first. My priority is cheap transactions, not speed usually. I've recently downloaded MyCrypto app. It has "Send ETH" in menu. In simple menu transaction fee is set in Gwei (23 to 67 now, couple of minutes before it was min 19 IIRC). If I click advanced, I see gas price (Gwei) and gas limit. IIRC gas limit automatically filled-in also changed from several minutes ago.

I've read theory: gas is measure of computation, bla, bla (https://ethereum.org/en/developers/docs/gas/). There is base fee now+tip (I guess MyCrypto reads advised base+tip from the network and puts in gas limit). In practice, where does one buys gas? From same miner who would include transaction in a block? Or some other place during processing? What would most probably happen if I try to "buy" gas (put gas price in transaction) of say 1, 0.1 Gwei? (I know Gwei is giga wei which 1 ETH divided by 10 to the 18 power)

Can I buy gas in advance cheap for processing fast next time?

TIA

Added: it seems ridiculous to me price could fluctuate several times (20 to 60) in matter of minutes. Seems it is just covert mechanism to prioritize transactions. But if transactions flow is more of less constant, why would anyone take transaction paying 1.5 times less than average? My only guess it that transaction flow is far from constant. So back to the question above: how long processing at 1 Gwei would probably take if submitted today?