The soldering paste is powdered solder mixed into flux. It flows very easily and bonds easily to material being soldered. You can apply a tiny drop with a toothpick to the pads on an emitter and it immediately melts and bonds to the pad neatly when heated. Just dip the ends of a piece of wire into the paste, then hold it to your soldering iron to tin the wire quickly and easily.
I know how to solder very well, but still go back to the soldering paste as it is so easy to use. The lodestar paste from DX/MF is a very good deal as well; local sources charge 2x-3x more for 1/10 of the amount.
I have been soldering for many years (Doing electronic development) and never been very critical about what brand of solder I uses, but it must have embedded flux cores and I will stay with leaded solder as long as possible (i.e. 60/40 or 63/37). Lead is not legal to do professionally in EU anymore. I very seldom need to apply extra flux
For soldering I mostly used thermostatic controlled solder irons.