There are two kinds of factory adjustable sights. The old ones and the new ones. Both have a square notch outlined with a thick white line. The old ones can best be described as "POS." The way to identify them are that the old style actually extend past the rear of the slide (!). This is their main (but not only) problem. They can be, and have been, knocked off during CCW or, in my case, while the gun was in a paddle holster in the console of my vehicle. If you are buying a used or reconditioned GLOCK that has these, factor in the cost of relacement.
The new style are not bad. I have not had to adjust them, every GLOCK I have bought came ready from the factory to shoot 155 gr .40 to point of aim, both fixed and adjustable.
Most people eventually upgrade to night sights. You can get 3 dot night sights from the factory, either mepro or Trijicons. I think the Trijicons run a little more. The most poular aftermarkey night sight seems to be the Heini "Straight Eight." I prefer the AO pro express, with a big dot for my G27 and G23 (trnslate for your purposes G26 and G19) and the regular dot for G22 and G35. There are a lot of people who will steer you away from these sights.
Anyway, good luck.