To find Ancient Debris, first you'll need a Diamond Pickaxe. If you mine the Ancient Debris block with anything less, it won't drop anything. The blocks can be found at level 8 through 22 (and only in the Nether), so you'll have to carefully mine in the Nether to find it.

What is the best y level to find ancient debris?

On average, Y-level 15 has the most ancient debris.

At what y level do you find Netherite?

Netherite, per its name, is found only in the Nether, specifically at Y-levels 8 to 22. That said, Y-level 15 has the highest probability of spawning Ancient Debris, the block that can be smelted into Netherite Scraps (which are then smelted into ingots).