Aerial drowned farm with the ceiling which is a glass bottomed pool

I'm in bedrock 1.18, survival mode. I don't like applying a video without understanding why it works.

I've read this wiki tutorial and I'm confused.

These 2 paragraphs made me confused: (it says 6 blocks and 13 blocks!)

Make the ceiling completely from glass blocks, with at least six blocks between the floor and bottom of the glass. 1 block of stone wall (excluding the slabs on the top edge of the wall) should extend above the top of the glass, to form a shallow pool that you must fill completely with water source blocks (no flowing water). You can build a temporary infinite water source off to the side to work with as you fill the pool.

Using a glass-bottomed pool for the ceiling takes advantage of the fact that drowned spawn under water but not on transparent surfaces, so they end up spawning on the platform under the glass. The water attenuates the sky light by 2, and each block of space under the glass causes further attenuation. One needs at 13 blocks of space (including the glass) under the water for the light level to be zero on the platform to let drowned spawn during the daytime. Drowned also cannot spawn on bottom slabs, but using bottom slabs instead of glass for the bottom of the pool makes the farm inoperable. The bottom of the pool must be glass.

Question

What is the content of layers? (I wrote my assumption below)

129-source water blocks (ceiling)
128-glass (ceiling)
[122-127]-space (6 blocks height)
121-flowing water (spawn platform)
120-dirt (spawn platform)

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