They’re similar. Generally welding is focused on fusing metals of similar makeup (I.e. steel to steel). While It does frequently (but not always) use “filler” but a good weld relies in the actual fusion of the separate metal pieces, basically melting the pieces together. The filler also tends to be similar in composition to the fused metals.

Soldering uses a dissimilar metal to join pieces and it doesn’t fuse the pieces together.

There’s also brazing, which is somewhere in between those two.

But, yea, honestly I generalize it all into a hot glue gun for metal :-P