Hello, I was wondering why bassinet mattresses need to be no thicker than 75mm but it’s safe to put a newborn in a cot where the mattress thickness is greater than this eg 120mm. Thank you!

The thickness of the mattress is related to the specific items & the height of the sides.
A cot has much higher sides than a bassinet so height of the mattress can be greater.

Product Safety recommends the following
With the mattress base set in the lower position, the cot sides or end need to be at least 500 mm higher than the mattress

https://www.productsafety.gov.au/system/files/Safety alert — Cots.pdf

Ensure the sides of the bassinette are at least 300 mm higher than the top of the mattress base.
Ensure the mattress fits well and is firm, smooth and no more than 75 mm thick.
https://www.productsafety.gov.au/products/babies-kids/kids-furniture/bassinettes

Red Nose recommends using a mattress that is firm, clean, well-fitting & flat.
If a mattress is too thick, it may not then be firm.
https://rednose.org.au/article/what-is-a-safe-mattress