Searched, learned quite a bit.

Here's my specific case:

Run from the main box to detached garage ~30ft

The garage is pretty small (20x20), and pretty old (~1920) with unfinished interior walls and somewhat deteriorated clapboard siding

In the garage I'm planning on the following:
--220V 40A (minimum) circuit for the welder, as future welders might be bigger
--A 110V circuit for something like 6 receptacles 4 small lights.
----Receptacles will run assorted power tools and an air compressor (which I guess is a power tool as well)
----These would be GFI as it's basically an "outdoor" installation, right?
--Probably a total amp draw in the 60-80A range, max.
--It's not the end of the world to me if the breaker occasionally trips, as long as it's not a safety issue

Do I need a separate sub-panel for the 110V circuits, or can I branch the lighting off one side of the 220V and the receptacles off the other? This would need a breaker at the main box that monitors current across the 220V leads as well as each 110V circuit... is there such a thing?

What do you recommend to do for the run to the garage? I was hoping to use UF 6/3, with no conduit. (buried 24" down with a warning tape 12" down, doesn't go under a driveway or anything) Would you recommend otherwise?

Whatever you would recommend for supplies, where would you recommend to get them? It seems the big chain stores don't have much of the heavier stuff. Most other electrical suppliers are wholesale only. Anyone online or nation-wide that you'd recommend? I'm located in Los Angeles, CA.

Thanks

GFI protection is needed for all 120 volt receptacles in a garage because of the use of power tools, not because it is an outdoor location.

I do not understand your subpanel question. A subpanel will give you both 120 and 240 depending on the branch circuit breakers used, 1 pole for 120 volt and 2 pole for 240 volt.

#6 UF cable is only good for 50 amps. I would use 1-1/4" PVC Sch 40 conduit and fittings, along with 3- #2 aluminum XHHW wire and 1- #8 THHW or XHHW copper for the equipment ground as the feeder. Or you can use #2-4 wire URD cable, also called trailer cable. This will need conduit sleeves where exposed above ground and in the building.

Be sure to know how to terminate aluminum wire, do a search here in this forum, I list all the details several times.

The PVC conduit and fittings, wire or cable, subpanel is all available at the box hardware stores. If there are any, a small or independent supply house may sell retail, such as CED, Many of the large chain houses, such as Graybar, Wesco, will only sell wholesale, and as they should. You may try Grainger, but I think they may only wholesale too. I am sure there are Web suppliers, but not familiar with any of them

Then use a 100 amp rated MLO panel, with 12 or 16 circuit, and a 2 pole 80 or 100 amp feeder breaker back at the main panel. You will need a separate equipment ground bar in any subpanel you install.