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.
For an Intel i7-11700, CPU temperatures of 40-60 degrees when doing standard operations (Firefox opened with 30 tabs, Dolphin, Konsole, a few other programs) seem standard. You should reach 35-45 degrees when completely idle (with a summer outside temperature). More is not normal and can indicate a CPU cooler issue or ventilation issue.

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.

Modern CPUs implement frequency scaling, which means they run at higher clocks under load and run slower when idle or under light load.
In the Linux kernel, software support is implemented in the Power management and ACPI options -> CPU frequency scaling sections.

Modern Intel CPUs (recent Core models) use the P-state driver. Documentation is available here.
This driver only support two governors: performance and powersave. Building other governors into the kernel is useless, they won't be used.
The performance governor produces high CPU temperatures that generally send the fans to high speeds, while powersave produces temperatures lower than 50 degrees.
You can switch the current used governor by using the cpupower program:

cpupower frequency-set -g powersave

cat /sys/devices/system/cpu/cpufreq/policy*/scaling_cur_freq
cat /sys/devices/system/cpu/cpufreq/policy*/scaling_governor

It is recommended to update the CPU microcode at boot. With initramfs it can be done easily; add the initramfs USE flag and emerge sys-firmware/intel-microcode. It will automatically copy an intel-uc.img file to /boot. GRUB can then load this file via an initrd line normally.
It's also possible to update the CPU microcode without initramfs support. In that case the correct firmware must be identified and built into the kernel. See this page for details.
In any case, you can check if microcode was correctly loaded by looking at dmesg (should be the first log line).

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?).