After finding an anomaly, Suvi will sometimes say that this planet is heavy in a specific resource. I am then able to scan the entire surface, but the scanner doesn't seem to change. Will I always get the same amount of resources independent of where I scan? Is there any indication that sending a probe in a specific area is better than another?
There's something to scan for if and only if Suvi says there's something to scan for. The "Activate Scanner" button will also glow if there's something to scan for, although it sometimes glows on an anomaly I scanned from system view, even though you can immediately click on the resource to gather, without having to actually scan.
Where you launch the probe matters. If you launch it in the wrong spot, you'll get nothing at all, but can continue to look for the correct spot.
As you scan, the scanner changes to show you where to launch the probe. An orange arrow will show up around the scanner reticle. Move the reticle in the direction the arrow indicates. The scanner reticle will switch from orange to green when you find the spot.
The planet rotates faster if you don't have the scanner active. I usually activate it to get a general idea of the direction, then turn it off and rotate in that direction until I see something, like a bright light or a dark mass. I then reactivate it and launch the probe.