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I like to weld.

Types of transfer of filler metal

Short Circuit Transfer (SCT)
feed wire touches molten metal pool, which kills the plasma arc. The surface tension of the molten pool pulls away from the feed wire, which restarts the plasma arc. The cycle repeats.
Globular Transfer
feed wire never touches the molten metal pool. The feed wire melts in the plasma above the molten poll. A single, large droplet of metal collects at the tip which eventually drops into the molten pool. This requires less power than Spray Transfer, but the quality and penetration is not as good. The droplet size is irregular and the position it falls into the molten pool will vary from the center.
Spray Transfer
feed wire never touches the molten metal pool. The feed wire melts in the plasma above the molten pool. The magnetic field pulls the droplets off the feed wire as a fine spray. Requires higher power giving more heat and a larger pool.

welding stainless to mild steel or iron

This is pretty easy. There is nothing special you need to do. Use regular Argon/CO2 75/25 (C-25) shielding gas and steel wire. I do this mostly for welding fasteners to steel pipes. I've also filled holes in stainless containers (I didn't have pure Argon or stainless wire). The welds are not stainless, but they are good and strong. The hole filler welds in stainless containers were exposed to a constant wet, acidic environment and the welds did rust, but for the purpose it wasn't a big deal. The heat doesn't seem to depassivate the stainless steel.

welding stainless with Argon/CO2 75/25 (C-25) shielding gas

For a home shop it can be expensive and a hassle to have to deal with different mixes of shielding gas. The most common mix of shielding gas in Argon/CO2 75/25 (sometimes 70/30). It's best suited for mild steel, but it works fine for stainless steel. For most applications you don't need Trimix (He/Ar/CO2) shielding gas.

I have been using ER308L stainless steel wire.

welding aluminum

I attempted to use Argon/C02 (C-25) shielding gas with aluminum. This attempt was a complete failure. It wouldn't even work in an emergency.

websites with welding info