A detail picture or table showing what the symbols mean in the display in Age of Empires 2

Like what does the 12+4 mean in the knife symbol, and how is it different from 16?

It is the same damage. 12+4 is simply added together and the total output is 16. The number right to the + is comming from unit upgrades.

Some units also have a red circle symbol (I think it's for ranged units). What do the numbers on that symbol mean?

The number next to the circle stands for the units range. Either take the number as it is and compare it to other ranged units to get a feeling for how much is what. It's been a while but I think it represents the tiles, that you can also see whenever you open the editor.

Also, in the scenario editor I think they show the stats for these units a bit differently [...]

The difference might be because of the huge amount of different civilizations, that each have their own bonuses. Or the units are from the campaign. I wouldn't pay that much attention to that.

I think your confusion comes from the hidden bonuses. If you checkout the damage stats on a Pikeman, you ll see that it does very little damage. But if you see it hitting cavalery, it does huge amounts of damage. The reason for that is there is a fixed value added to the Pikemans damge, whenever it hits a cavalery unit. So the damage would be something like 3 + 20 against cavalery and 3 against the rest. Another good example is the Ram. You ll see that it only does 2 damage. But when you see it hitting a building, it actually gainst +200. There are many hidden bonuses like that. They are done this way to have you finding out the effective counters by playing.

Please don't pin me on the exact numbers, but this is the way the system works. If you want a fun way to get into the AoE2 mechanics, you might like the youtube channel called "SpiritOfTheLaw". He goes very in depth into the mechanics in a very fun way.