Sensors

CPU temperature

To get CPU temperature, kernel modules are needed. I2C support in particular is generally required (compile I2C_CHARDEV).
In addition, the correct modules corresponding to your chip should be added in the Hardware Monitoring support and I2C sections. One good technique is to compile everything available, then run sensors-detect, which will find the correct module. Then you can delete all the other modules and rebuild the kernel.
For a recent Intel Core CPU, compile in the SENSORS_CORETEMP module (Device Drivers -> Hardware Monitoring support).
In user space, you need to emerge lm-sensors.
A KDE widget (plasmoid) is available to display current CPU temperature. You need to emerge ksysguard with the lm_sensors USE flag in order for it to work. It's not very beautiful but functional.

lm-sensors is also needed, along with some kernel modules (same procedure as for CPU temperature above).
You may need to add the acpi_enforce_resources=lax command-line kernel boot parameter for some ASUS motherboards (like the ROG Strix Z590-I).
You can then get the current fan RPM by running the sensors program.

On some ASUS motherboards you may need to build the Asus atk0110 ACPI module. You don't need anything else but this module is not detected by sensors-detect; you must load it manually in the configuration file, see below.
If you use the Asus atk0110 module, enter the following in /etc/conf.d/lm_sensors:

MODULE_0=asus_atk0110

Fan control is better done in the BIOS. Normally it will allow you to set a fan profile, which associates a given CPU temperature with a given rotation speed. Usually you will want to keep rotation speed low to minimize noise, even if the CPU gets hot. In particular, running the fan at full speed produces a lot of noise at least on Noctua fans.
CPU temperature should be less than 45 degrees while idling, 75 degrees under normal load, and 95 degrees under heavy load (gaming).

Modern fans implement this interface, and in theory this makes it possible to control the fan speed from the OS rather than the BIOS. In practice, I think it's hard to do so and I was not able to even obtain metrics via PWM.
Kernel modules that might be needed for this include Device Drivers -> Pulse-Width Modulation and Device Drivers -> Hardware Monitoring support -> PWM fan.

Some modules in the ACPI section (mostly for CPU frequency scaling).
Some modules in Device Drivers -> Multifunction device drivers (in particular the Intel LPSS modules?).
Some modules in Device Drivers -> X86 Platform specific drivers (Intel ones?).