Back in the days Before Covid (BC) there were large coin shows and PCGS used to have coin grading seminars. I hope to go to one someday. I have learned a bit about it from a couple of the PCGS PowerPoint presentations on youtube. https://www.youtube.com/c/PCGScoin
They also Attribute Mint Errors and Varieties with the same techniques mentioned before. I saw a picture over at NGC where they have a library of reference books for use in attribution. And one important factor is talking with their co-workers for verification of the attribution.
DarkRage666, asked: "How do the grading companies grade coins? What tactics do they use? What instruments do they use? Too broad a question. Tactics? Some graders start from the back of the box. I use a 7X hand lens and stereo microscope (mostly at 7X). I also us two different light sources, rarely a specific gravity rig, and conservation stuff. Otherwise: Conder101, posted: "Eyes, consistent lighting, sometimes a low power loupe, knowledge and experience.
Did anyone notice that on occasion a coin is looked at again while still in the flip. That simulates how it may look in the plastic slab.
At HGC they use magic (Hogwarts Grading Company) but it's really trixie and those leprechaun coins can up and disappear in a second if you don't have the room sealed properly.
I was told that in the bulk department at one TPG, when someone submits the fresh from the mint stuff like commems and proof Eagles, they don't even take them out of the mint capsule to grade them. They get removed from the capsules later in the process, before holdering them.
Depends on who told you that and what company they work for. I have it on very good authority that at one major service they flip any of the "liners" to see if it gets the higher grade.