Can I use this flux for soldering SMD components?
I watched tutorial videos for SMD soldering, and I saw they all put flux on PCB before soldering, is that really necessary?
It can help, personally I usually simply tin the pad and then solder the component to it.
But you MUST use the correct solder, the correct flux (if you're using extra flux), and a proper soldering iron with a very fine bit. If you're soldering multi-pin IC's then flooding the pins with flux allows you to drag a spoon shaped bit full of solder across all the pins, and solder them all in a single operation - Pace, for example, make special bits for that purpose.
The pictures you posted of the remote control looks like you used one of the irons in JimB's picture above
This is the sort of bit you might be looking at for simple SM work?
when i worked for a prototyping/test fixture company, we actually did use ZnCl for a set of test jigs we made for Raytheon. all of the terminals on the test jig were silver plated, and the specification was to use silver solder with ZnCl flux. ALL of the flux was then removed, the organic material with TCE, and any salt residue with hot water. if you ever run into the ceramic terminal strips with silver plated onto the ceramic, silver solder and ZnCl flux is used on those as well... again a good solvent (alcohol is more common these days) and hot water to remove any salt residue (there is a US Air Force Technical Order describing all types of soldering in use, with prescribed techniques for each, and this TO is used by the other 3 services as well). it's use is rare in electronics, but not nonexistent.That flux contains Zinc Chloride which is corrosive and not for use on electronic equipment.