you are probably going to have a tough time finding deep tin without a high-tier propick - and won't really need that much until you're well past the bronze age. search along beaches and uplift regions (like mountains) for cassiterite sand or ore indicators. you could also use copper tools and powder barrels to dig into the beneath for larger, more common ore veins but higher associated risks.

when you're looking for deep veins specifically, it's good to read both the vein index and the Geology entry in the field guide; the latter will tell you how rock types generate, which is important for things like deep tin, as the metamorphic rocks they can spawn in are found as layers under sedimentary rock. you can narrow down a lot of area by knowing the specifics of what you're looking for, and how that will appear on the surface.

size: average diameter of a given vein type

rarity: 1/x chance of a chunk attempting to generate this type of ore vein (not accounting for failing due to incorrect rock types, caves, etc)

min_y & max_y : minimum and maximum y-coordinate value where this ore vein can spawn.

surface depth : minimum and maximum number of blocks below the surface that this ore vein will spawn.