After putting the rocket on the launch pad the propellants are filled.
But before that how to maintain the lower temperatures of the tanks so that when filling the propellants don't boil off?
Space Exploration Stack Exchange is a question and answer site for spacecraft operators, scientists, engineers, and enthusiasts. It only takes a minute to sign up.Sign up to join this community
Short answer, they let some boil to keep the rest cool and also 'waste' some to cool the plumbing and tanks before loading.
The bulk cryogenics are made by compression, not cooling and from that point on normally the liquid is kept liquid by evaporation during shipping and storage.
This can become a complex engineering problem for large rockets to handle the boiled off (but still very cold) gases and keep them from reacting/exploding or just icing up other parts of the launch pad/rocket or people.
Edit: in the case of supercooled/densified propellant you need to use a cooling system on the pad and circulate the fluid. This cooling system will use a gas with a boiling point lower than your target temperature, possibly Helium.