Big doughnut-shaped tank in the spare wheel well, bit of plumbing to the front (copper in old installs, plastic on newer ones - my "old" Rangie has plastic pipe because I refurbed it about seven years ago), and a thing like a diving regulator that lets the engine "suck" gas through after boiling it off using engine coolant, and feeds it into the throttle body. More sophisticated ones use solenoid injectors that work alongside the normal petrol injectors and provide a more "direct" flow of fuel and better performance. On my oldschool install there's a stepper motor that adjusts the fuelling based on what it reads from the existing lambda sensors. When you switch to gas, a pair of modules (four cylinders each so a V8 needs two) switch some resistors in series with the petrol injectors so they cannot fire but the ECU still sees continuity, cutting off the petrol, and the gas solenoids switch on allowing propane through.

To fuel up you just go to a filling station with an LPG pump (getting harder to find here unfortunately) and instead of poking the nozzle down the filler neck and holding down a trigger on the filler gun, you plug in a hose with a fitting like a BNC plug the size of your wrist and hold down a button on the pump until you've put enough in. It clicks off when it's full.

No difference in performance (in theory you could adjust it to get more because it's the equivalent of 115RON fuel!) and a bit more fuel consumption, but roughly 2/3 the price of petrol.

LPG (liquified propane gas) stations are slowly on the wane but still common enough for practical use in Germany.

Common enough and cheap enough that I still feel a bit prescient for buying a 2017 Ford Focus wagon with about 30k km on the clock that was factory modified for LPG, in August 2021, for about half what the original owner paid.

In Poland, these are about as common as regular gas stations.