What is the best way to see fine solder joints with a digital scope?

The trick is to not work at full zoom. Nobody soldering under a scope runs it at max magnification the whole time. Drop down to the lowest setting while you've got the iron or hot air in play, enough that you can see the pad plus some of the surrounding board. The wide view is what tells you where heat is going, wheather the component is shifting, and if flux is bubbling somewhere you weren't watching. Then when you're done with a joint, crank it up just to inspect the result and drop it back down for the next one. Trying to do the actual heating at high magnification is what's making you nervous, and rightfully so, because at that zoom you genuinely can't see the tip approaching the joint until it's too late.

On the lighting question, glare off shiny metal is mostly about angle, not brightness. Ring lights that sit straight over the board are the worst offenders because the reflection bounces right back into the lens. If your scope's light is adjustable, turn the intensity down and angle the source off to the side so the solder pads show a soft sheen instead of a blown-out white spot. A shiny pad that reflects evenly and cleanly is usually good, whereas a dull, gray, or grainy pad means old solder that needs reflowing or cleaning up before anything new goes down.

Flux is your friend here too. A decent dab of flux on the pads before you start makes the solder move visibly instead of just sitting there, so you can actually watch it melt and flow rather than guessing. Wet solder also reflects light differently from dry, so the change in appearance is your real-time confirmation.

If you can, run the scope's output to a monitor instead of looking through the eyepiece the whole session. Your neck and eyes will thank you, and it's easier to glance between the board and your tool when the image is up on a screen in front of you.

Congrats on the upgrade, you're gonna wonder how you managed with a loupe. One thing that helped me early on was grabbing a pile of dead boards from old electronics and just practicing drag soldering on them before touching anything I cared about. Cheap way to build the muscle memory of moving your iron while your eyes stay on the scope. Also don't underestimate how much a steady rest helps, I brace my pinky or wrist on the desk or the board edge, otherwise every tiny hand twitch looks like an earthquake at that zoom.

For BGA work specifically, get the whole board warm before you aim the nozzle at one spot. I do a fair bit of game console repair and I run my hot air around 320 with a preheat underneath, the joint hits reflow in seconds and the component lifts off cleanly instead of the pad. Cold board plus concentrated heat is exactly how pads tear off. Also check your flux, if it goes brown and crusty before the joint moves you're way too hot or too close.

I’m trying to use this microscope for some older industrial boards I found, and I keep getting worried about the heat spreading too far. The scope helps me see the specific pad, but I’m relly unsure how to know if the heat is traveling into the nearby trace or components before the solder even starts to flow. It seems like a tight balance. Do people ussually try to preheat the whole section first or is it just one spot at a time for these smaller chips?

I’ve read a lot about the light settings but the heat distribution is what keeps me awake. I don't want to blow a trace right next to the area I'm trying to fix.

I’m hesitant to agree with the idea of dropping to the lowest zoom when the heat is actually applied. I find that if you set it too wide while using the iron, the tiny pad becomes too small to resolve properly, which is why you can't tell if the solder is actually beggining to flow. You need enough magnification to clearly see the pad geometry and the tip contact, even if you don't view the whole board at that specific moment. The lowest setting is better for pre-inspection, not for active heating.

Update: turned the zoom way down like the first reply said and it made all the difference. I can see the pad plus the traces around it now, so no more melting surprise. Also started warming the board underneath before hitting the joint with hot air, wich the console repair tip suggested. First rework came off clean. Practicing on junk boards this weekend before the real one.