Tools
Y levels from this decade, not 2013.
Diamonds want the bottom. Iron has a mountain habit. Ancient debris is a Nether problem at about Y 15.
Coal
Overworld · best Y 96
A second, even band runs from Y 136 up through mountains. A stone pick is enough.
Y -64Y 0 to 320Y 320
Iron
Overworld · best Y 16, or Y 232 in mountains
The useful underground band peaks at Y 16. Tall mountains have a much denser band peaking near Y 232.
Y -64Y −64 to 320Y 320
Copper
Overworld · best Y 48
Dripstone caves grow bigger blobs. Stone pick or better.
Y -64Y −16 to 112Y 320
Gold
Overworld · best Y −16
Badlands add a lot more gold between Y 32 and Y 256. Nether gold ore is a different block and is common across the Nether.
Y -64Y −64 to 32Y 320
Diamond
Overworld · best Y −59
More common as you go down. Air-exposed diamonds are rarer, so a branch mine beats wandering a cave. Deepslate replaces stone below Y 0.
Y -64Y −64 to 16Y 320
Ancient debris
Nether · best Y 15
The Nether, not the Overworld. The dense band is around Y 15. Beds or TNT are the usual way to clear it. Don’t do that next to your portal.
Y 0Y 8 to 119Y 127
The bar is the band where that ore can show up. The mark is the Y people actually dig. Press F3 on Java, or turn on Show Coordinates on Bedrock. The trip that uses the diamond line is the starter checklist.