A couple of questions, please.

For a shotgun to be used for waterfowl and upland birds, what are the weight differences between over-unders, semi-autos and pump guns? Any recommendations?

Again for a gun used for waterfowl and upland birds, 12 or 20 gauge? Whats the difference between (say) 1 1/8th oz out of a 12 or a 20 ga.?

Finally, I haven't shot since lead was outlawed. What differences does steel make?

My auto and O/U both are 7.25 lbs, 3" chambers.I find this a good weight for waterfowl, upland,sporting clays.There are lighter and heavier ones.
There are a number of waterfowl loads available ,steel, bismuth and others New guns will be OK for steel but check for older ones.Steel has somewhat less range but do some research for the appropriate range for any shot .I've seen TV shows that recommended absurd ranges !!
Whatever you pick make sure you pattern the load, it may be very different than you assume !!