https://wiki.ion.radar.tech/tech/lightning/lightning-network
tl;dr: you and the counterparty negotiate a series of transactions that represent the final closing balance. As funds are sent/received the transaction is updated. There's various mechanisms in there to prevent a misbehaving peer from broadcasting an older transaction, running away with the funds, or having your funds frozen if they ghost you.
It's not meant for buying coffee, for those you would use layer 2 or centralized entities like Paypal, etc. The blockchain contains every single transaction ever made forever, people in 100 years from now don't need to be aware of every coffee transaction, that just wouldn't be efficient.
That's the current goal of the developers of these systems, anyway.
It's kept trustless, because people trying to cheat can get their bitcoin stolen. This is done using fancy bitcoin multi-signature scripts. Closing a Lighting Channel using an old state (ie: being dishonest) reveals keys that can be used by the other (honest) party to take your funds.
> That's because you're looking at it in terms of USD, when you look at it in terms of satoshis per byte, it's actually much lower [1][3].
This is meaningless to me. Why would I care about transaction costs in satoshis per byte? I thought bitcoin is supposed to be a currency that replaces USD. At the current transaction fees bitcoin is not usable as a currency unless you're buying very expensive items.