In Minecraft java edition, what is the main contributor to using cpu resources.

Mob natural spawning and entity ai

Also redstone if your players like to use that

Mob spawning? Is it due to it trying to spawn mobs every tick or is it the actual mathematical process of evaluating valid spawn conditions.

I highly suggest take a sample with spark yourself and see
There are a number of factors that make it heavy, obviously one of them is trying to spawn mobs every tick, but also looking for valid positions, biomes, bounds, etc

The factors I mentioned above are the ones that affect a "normal" gameplay
If you add custom mechanics, or datapacks, or mods; or if your players are those that build huge farms, have a ton of automation, try to exploit stuff like the rng, then obviously the "most impactful process" will be a different one

For example, take a look at this guy's post, only 3 people playing normally, but they installed a ton of datapacks, there's no trace of mob spawning, entity AI, chunk generation, or any common lag source impacting performance in their server.
Datapacks by itself are already a pain for performance, you kinda get an idea what happens when they added 20 of those
https://forum.admincraft.wiki/post/1440418442674049095

Ah don’t worry, i am fully aware of how expensive datapacks are compared to plugins, /execute is a heavy command to run, even at Reduced frequencies.

I will never plan on deploying datapacks into production units, ever.