Multiple clarifications -
Antihistamines do not directly dry up your runny nose/post nasal drip. Rather, they reduce the histamine response, which is helpful when something is caused by allergies. It's usually not that helpful with an actual cold (but allergic rhinitis is often mistaken for a cold).
Sudafed is what the parent was referring to with Pseudophedrine; a decongestant. This works by shrinking blood vessels, causing swollen passageways to open back up. It doesn't actually do anything to reduce the runny nose or post nasal drip, but by opening the passages more can help them drain more efficiently, preventing that feeling of congestion (hence the name).
Guaifenesin (Mucinex being the name brand) is an expectorant; it causes you to generate more mucus, and reduces the viscosity, allowing you to cough/sneeze/etc your mucus/phlegm based congestion out more easily.
Otherwise I totally agree; it is worth understanding what each of these do so you can pick and choose what you need. Nyquil includes DXM (dextromethorphan) to reduce cough severity, but the acetaminophen is not helpful unless you have a fever or headache, and the Doxylamine Succinate and Phenylephrine (both antihistamines; the Doxyl is added to Nyquil because it's also a sleep aid) aren't particularly helpful unless your cold symptoms are actually allergy related.
Better to buy DXM separately usually; fewer side effects, cheaper, and you can pair it with what else will help you (if you need something to help you sleep, you can add Doxyl or Diphenhydramine if you want; YMMV as to how effective they are)
Don't take Guaifenesin before bed (the increased mucus/phlegm production will make it harder to sleep), but it's good during daytime.
A common side effect of Sudafed is trouble sleeping; if you don't have this side effect it can be helpful in reducing congestion while you try and sleep, but if you do have this side effect, obviously, don't take it before bed.