So I noticed something really weird : I have like 100c temps on houdini when doing simulation, and as soon as I look at something else like discord or google chrome, the temps come back to normal like 30-40 even if I'm doing simulations.. No idea what can cause this but it's pretty annoying.. Anyone has any idea?

spec: i9 13900k, 64gb ram ddr5 6000mhz, and a 4070ti

Give Prime95 [en.wikipedia.org] a test. You may have to reapply your thermal paste.

Most likely the thermic paste as mentioned at that temperature.

Also make sure the heatsink is attached flat and even.

So I added themic paste and made sure the heatsink was attached flat which.. Turns out it's quite hard to have it not flat.. And I still have crazy temps only while watching a sim in Houdini.. Any other software is fine and any games, any rendering engine.. I dont get it..

Scratch that, I rendered a random cube and made it insane to render in arnold settings, and again only while watching the render, I have crazy temps and the cpu goes to 100%.. When I stop watching but the render is still doing it's thing, the temp goes back to normal like 30-40c. I'm lost

It's normal for the 13900K to get very hot when pushed to 100% by rendering or simulating.
Otherwise it's very efficient. Games and daily OS tasks can not really push it that much.

Motherboard manufacturers push it even further. There should be a BIOS setting to fix this (a little).
In ASUS motherboards it's called 'ASUS Multicore Enhancement' - 'Enforce all limits'.
It's still gonna get toasty though.

Don't know about watching the render vs not.
Maybe there is setting in the OS which throttles the background apps? Like a power saving mode?

100 degree celsius is max for this proc.

Your text to link here... [www.intel.com]

I’ve struggled with other CPU:s but not that one.

Also I don’t know what to respond regarding ”crazy temps”.

Probably read the manufacturers recommendations regarding settings and cooling requirements.

air circulation is important.