It doesn't. They're almost always open collector/open drain signals, with the pull-up on the host (PC) side. No power needed, if you've got mechanical switches doing the commutating!
(Of course those aren't so reliable over the long term, so transistors are usually used. But swinging a base or gate around is all that's needed, and that is easy to do with almost any available power source. Like the main power for the fan!)
Generally the fans are powered with a steady state DC source and the PWM is just logic signaling. If the fan is powered by a PWM source the tach output will not be a clean square wave (but could still be reconstructed, if the PWM frequency is sufficiently higher frequency than the fan's rotational speed).
I can't remember the last time I encountered a motherboard that didn't offer you the choice between DC voltage and PWM for fan speed control. Both are still mainstream options; CPU heatsink fans are almost always 4-pin PWM fans but case fans are very commonly 3-pin DC fans. AIO Water cooling kits also commonly use 3-pin fans, especially when integrating their own fan controller or multiplexer so that they only need to occupy one or two fan headers on the motherboard.
(Edit: if you mean that in practice, the motherboard approximates DC voltage control using a PWM-based method, that may be true, but it's beside the point; a 3-pin fan is missing the control circuitry that a 4-pin PWM fan uses, and the 3-pin fan's behaviors like turn-on point will be specified in terms of DC voltage not PWM duty cycle, and you can't drive a 3-pin fan with the PWM signal intended for a 4-pin fan because the voltage and current supplied are both far too low.)
(Of course those aren't so reliable over the long term, so transistors are usually used. But swinging a base or gate around is all that's needed, and that is easy to do with almost any available power source. Like the main power for the fan!)