lats

where are they
when i do a pullup and i feel a stretch right under my armpit am i exercising the lats or no
or is it when i squeeze my shoulder blades together at the top part of the pullup

yes, under the armpit is the right spot

Just spam pull ups and rows you'll feel them soon enough

Below the armpits is correct, near the shoulder blades is more traps.
If you're new it does take some time to feel the lats usually but just try to think that you're pulling your elbow close to your body when you do the pulldown. For rows I'd say try chest supported rows if that's possible. Despite all this, different things work for different people. Keep adapting your plan based on how your body is feeling.