Low temp silver solder is perfectly adequate for pushrods, cabane struts and just about anything else in a model. While a solder gun will work for it, I find it best to use a propane torch for the job, as good even heat is key to a good joint (as is thorough cleaning and good flux). High temp silver solder isn't necessary for most of what we do. But if you want to learn a new skill, go for it!

You currently have (I think) what we refer to as a stick welder, where you put a flux coated electrode, about a foot long, in an electrode holder, strike an arc, and manage feed rate, arc length, etc manually. Every couple minutes, you install a new electrode and continue on. This process is good for mostly heavier steels, though some fine work can be done, it's tricky.

Mig I believe stands for Metal Inert Gas (could be wrong here, but it's close). The welder contains a spool of welding wire, usually dozens of feet. The wire is fine, about 0.030", and is mechanically fed by a motor at a predetermined rate thru the cable (it's hollow), and out a gun. The welder is trigger operated, pulling the trigger turns on the welding transformer and starts the feed motor. Striking the arc is effortless, all you gotta do is hold the gun steady and keep a consistant lenght of exposed wire. In true mig welding, there's no flux, shielding is provided by an inert gas (co2, argon, etc. are used, different ones for different metals). There's a variant of Mig used mostly by hobby welders (like myself) called flux core. This is good for mild steels (most of what you or I use for shelves and the like). You use the same mig welding machine, but change the wire inside to this flux cored wire (it's hollow and full of flux), and you don't need the shielding gas. Works very well, but there's more spatter and there's slag left to be cleaned. Tru mig welding leaves no slag. Mig is great for working with sheet metals, as well as structural metals, down to about 20ga, and depending on the machine, up to as thick as you like. My small unit is good to 1/4" thick plate, plenty for my needs. It operates on 120v wall current, a bonus for me.

Tig I believe is Tungsten Inert Gas, and is actually closer to oxy-acetylene welding, in my mind. The welding machine provides only an arc, not the filler material (the rod, if you will). The welder strikes the arc with the electric torch, and then feeds a separate electrode into the arc with his other hand. Shielding gas is used, like with Mig, and is provided by the electric torch. This process gives an experienced welder amazing control and the ability to produce perfect welds with no slag and little heat damage to the surrounding metal. I've seen experienced TIG operators weld a pop can back together, water tight!

That's a quick answer. Lots more than that to know....

J