Depends on the load it will carry which you failed to list.
i just left lowes an they told me to run 100 amp to my building i would need wire size 2-2-2-4 alumninum and run it in conduit the lowes around here does not sell copper wire that size
If the run is less than 100' use 4 AWG copper.
Depends on the size of the circuit which you did not list.
For a 75 amp circuit with a 100 ft run, you would need a #4 AWG copper wire to ensure minimal voltage drop and to meet electrical code requirements. Using the correct wire size helps maintain the efficiency and safety of the circuit.
12/3 will work.
Depends on length. The more length, the more resistance.
I would recommend you use AWG # 2 copper or AWG # 1/0 aluminum for that distance and amperage.
12 gauge underground wire or if you think you will ever add any devices to this circuit use 10 gauge.
The wire size depends on the amperage, not the voltage. The fact that the welder runs on 440 volts does not affect the wire size! You need to ask this question instead: What size wire do you need for a three phase welder on an eight foot run if the nameplate amps are 45A?
45
For a 100' run from the panel to the RV using a 30amp breaker, you should use 10 gauge wire to ensure safe and efficient power delivery. This wire gauge is appropriate for the distance and the current capacity of the breaker.
You size wire by the current. Various insulations are rated for voltage, temperature and the medium where the wire will be run.