If you're becoming confused about the 'rules' of soldering - it's because you are overthinking things. You need to just understand the basics ... and then learn by doing.
Basic No 1. Flux.
Note: In the following, I will be talking about using a flux-cored solder, with no external flux - for one very specific reason ... you will get to understand the basic process much better. This will mean that, when you come to do your own soldering, with or without additional flux, you will know what's going on - and have a better idea of what you can and can't get away with.
Solder bonds to clean metal - not dirty, oily or oxidised surfaces.
- If the object to be soldered is particularly dirty, mechanical cleaning may be necessary. For critical applications - such as aerospace - mechanical cleaning may be required on any surface to be soldered, no matter how clean it may look, to ensure it is as free from contaminants as possible.
- Even freshly cleaned metal will oxidise in a short time - and heating the metal will accelerate this
- Flux is used to dissolve these oxides and allow the solder to bond with the metal
Now for the biggie....
- Flux will deteriorate from the heat of soldering. Since it is important to have it working on the surfaces to be soldered, a lot of the 'rules' circle around this very specific issue. If you keep just this one thought in mind, then a LOT of what might be confusing to you now will become much clearer.
Putting a blob of solder on the tip of an iron and then transferring this blob to the workpiece with the intention of using this blob of solder as the jointing material is the absolutely WRONG thing to do. If you see people who have been soldering for a while doing this ... look more closely. You will probably find they are simply adding a blob of solder that they can bring to the workpiece to allow better heat transfer from the iron to the work (The molten solder wraps around the joint and provides more surface area for heating - even if that solder doesn't 'wet' the joint.) Once the joint is heated, you can apply flux cored solder where the flux removes the oxides and the solder wets the joint. (Yes, you will probably see some of the blob get sucked into the joint, but that wasn't the point.)
Heating the joint and applying the solder can be done in one of 3 ways - and how good or bad each method is is directly related to the degree of degradation of the flux.
1. Heat one side of the joint and apply solder to the other, so that the solder is melted by the joint, not directly by the iron. While this is best for flux preservation, the joint will be subjected to more heat - so you need to be aware of this and consider the risk of heat damage to what is being soldered.
2. Heat the joint and apply the solder to the joint and the iron at the same time. This gives quick melting of the solder and the flux is very close to where it needs to be.
3. Adding solder to the iron and then applying this to the joint. A bad practice to get into - but if you do find it necessary to do something like this (in drag soldering, for example) you are going to need external flux for a reliable result. With this method, as you carry the blob to the workpiece, that wisp of smoke is the flux burning up - so you need to have flux already on the workpiece.
Basic No 2. Heat.
You will need a certain amount of heat to complete a joint. This is a function of time and temperature. Too high a temperature can cause a lot of problems .. from flux vaporising before it can do anything to frying components. Too low a temperature is also bad if it increases the time it takes to do a joint. You might feel it's doing less damage, but you are just using the 'slow cooker' approach. If the iron is hot enough to melt solder, it's hot enough to do damage. Look at a few videos of people with some experience to see how long they take to do a joint - a lot of the time it's under 2 seconds.
Basic No 3. Tip - size and shape.
Use a tip on the soldering iron that is appropriate to the joint. Thick tips can hold more heat closer to the joint, so you can finish the joint sooner, but they can be clumsy. Also, use a tip with a shape that reasonably matches the surfaces being joined. Better surface contact will carry heat into a joint more quickly. This is why nobody here has much in the way of positive comments on round tips.
Basic No 4. Solder.
There's a lot to discuss here - but if you're just getting into it, a 60/40 or 63/37 flux cored solder is where I would start.
Basic No 5. Iron.
Some might complain this should be higher up in the list - but if you understand what is going on, you can do decent solder joints using a nail and a blowtorch. It won't be easy and it won't be efficient, but it can be done.
The main thing is, to get something appropriate to the type of work you will be doing - and if you're serious about electronics work, spend the money on a decent temperature-controlled iron.