Hi Leif
As you have suggested, there are products and methods.
Products can be used to implement different methods and the two are not mutually exclusive.
There are two basic methods -
1. Providing excess nutrients to ensure there's always enough nutrients. EI (Estimative Index) is popular and effective for many. Usually dry chemicals are used to provide macro and micronutrients i.e. NPK, and CSM+B or similar trace element powders. Sometimes an off-the-shef product(s) can be used to implement EI i.e. Tropica Plant Nutrition+ (TPN+), the ADA range, Easy Life range etc. Mixtures are also used i.e. dry chemicals for macronutrients, and an off-the-shelf product for trace elements (micronutrients).
2. Providing 'just enough' nutrients to ensure there's enough nutrients. This can be relatively difficult and relies on calibrated test kits to test for nutrients. One such method is PPS (Perpetual Preservation System). However, very rich substrates can be used to ensure enough nutrients are available. The ADA system is a good example of this. Liquid fertilisers are generally 'lean' and the substrate makes up for any lack in water column nutrients. Once the substrate becomes depleted in nutrients, more nutrients are required in the water column. ADA compensate for this with their Step 1,2,3 range, ECA, and Special Lights and Shade.
Mixing products is usually ok, with a couple of exceptions i.e PO4 and Fe. There are ways around though, as described in JamesC's All-in-One article here -
http://www.theplantedtank.co.uk/allinone.htm
The pros and cons of each method/products largely depends on your system and goal.
For example, for large high-energy tanks (lots of light, CO2, flow etc.) then EI or PMDD+PO4 with dry chemcals is likely the best value.
At the other end of the scale, for nano aquaria without CO2, then adding an off-the-shelf comprehensive micronutrient additive may be ideal.
For El Natural/NPT/low-tech systems, then relying purely on a soil-based substrate and fish food/waste may be ideal.+
Finally, there is no specific right or best way to provide nutrients to you plants. In my opinion, if whatever method/product works for you, then great! Try out different products and methods and see what works best for you and your tank(s).
I hope that helps.