Is there a knack to finding which class to target?

I’ve just begun using WordPress (coming from a strictly HTML, CSS, JS background) and am having trouble getting my websites looking exactly as I want them, so I have been delving into modifying the theme CSS using a child theme. However, I’m struggling to get to grips with knowing which classes to target as the class names in my theme (Astra) are not very descriptive.

All I’m trying to do is change the height of the header, but there are about 10 different wrappers/containers within the header itself and none of these actually change the header height when I apply the CSS to it. Is there something I’m missing? Does anyone have a set of steps they go through to specifically change one property in this way?

Thanks in advance!