Regarding weed and feed, there are 2 types and unfortunately neither work great.
Granular weed and feed is simply iron sulphate and urea (or nitrogen of some description - might be sulfate of ammonia). You put it on, don't water it in. Everything turns black (iron overload) weeds die off and lawn comes back. 12% iron, rest ammonia - I can make it up for 1/4 the price bunnings charge. All it is is a few % iron, few % ammonium sulfate (as an aside, this mix is sure to lower soil PH as both are acidic).
Literally 1950s tech.
The liquid weed and feed in the spray 2l bottles is literally what you bought concentrated today but with a couple of extras (bit of nitrogen and iron). Basically you are paying for it to be watered down. The main problem is its too watered down by the sprayer - the water washes the chemical off the leaves. If you have a lawn covered it weeds, it will help.
It is not a complete lawn feed all it is is nitrogen. You need phos and potassium (P and K).
Without offending anyone, weed and feed will turn a poor lawn half decent. But it won't do much for a lawn that is already decent.
You are better off buying a 20kg bag of a decent granular lawn fertiliser, then backing it up with a foliar application of liquid fertiliser like powerfeed.
