Why does the same swap cost me a different amount depending on when I do it?

I swap between SOL and USDC a few times a week, usually a few hundred dollars at a time.

The part I've never understood is the fee side. Some days it goes through for next to nothing. Other times the wallet wants noticeably more before it will confirm. I assume that's priority fees when the network is busy.

Is that worth paying attention to at my size, or is it just noise? Do you set priority fees yourself, or do you leave whatever the app picks?

And when you look at a swap afterward on an explorer, what are you actually reading? I can pull up the transaction, but I'm not sure which number indicates the actual cost of the trade.