Best resource to prep for coding interviews? Hint - Not #Leetcode

Embedded folks! Where do you find your practice? The questions we are asked is not the typical leetcode based, but more specialized C codes, like memory management, dynamic memory allocation, strings, bitwise manipulation, stack etc.

For the pure SW side folks, before you judge us too quick, these are hard problems which some of my SW engineering friends preping for FANGs could'nt crack.

Btw, i am done with glassdoor, geeksforgeeks, leetcode, quora etc.

I am specifically looking for "Hard" C programming questions for me to practice.

Edit 1: preping for senior software/Staff engineer roles.
Yoe -6 (with MS).