There are units that come with compressors pre-charged with an amount of refrigerant for the units and a lineset length of between X and Y. You can also buy linesets of different lengths that have all the ends pre-fabricated.
After installing everything, you open two valves on the compressor unit to flood the lineset and evaporator with the refrigerant. It's not perfect [the lines are likely to have some water vapor in them], but it works. (The manufacturers also charge a premium for these "DIY" versions, so if you can find a way to do the traditional charging, you can save some money [or apply that towards an HVAC tech to come out, do a quick vac the lines/leak test/charge].
After installing everything, you open two valves on the compressor unit to flood the lineset and evaporator with the refrigerant. It's not perfect [the lines are likely to have some water vapor in them], but it works. (The manufacturers also charge a premium for these "DIY" versions, so if you can find a way to do the traditional charging, you can save some money [or apply that towards an HVAC tech to come out, do a quick vac the lines/leak test/charge].