There are typically two different kinds of valves that both bypass the solar system in different ways. Typically a bypass valve is a standard ball valve setup so it is either 100% solar or 100% pool. A diverter valve is usually a three way valve that lets you control exactly how much water goes to the solar system and how much goes to the pool, with continuous adjustment between the two.
The drawing above is the most basic setup you can have, with a simple bypass valve. Using this setup you can get some control over how much water goes each direction. Inexpensive ball valves aren't especially good at this, the movement isn't smooth and the setting tends to drift over time, but they do give you some control.
There are fancier ways to control the system, such as the diagram on
this page at Hot Sun Industries. This setup adds a diverter valve, show motorized thought that isn't essential, and a second check valve. A three way diverter valve is more expensive than a ball valve, but it gives you much better control over the relative flow. The second check valve prevents the solar system from being pressurized when the diverter is set to not send any water to the solar panels.
Automatic pressure based valves exist, but they are usually setup for a fixed pressure, while you would need to tune one to a custom pressure for each solar install to have it work correctly. I have never seen them used that way for a solar system, though it would be possible in theory.
Tuning the pressure in the panels isn't that difficult with a manual valve. Typically for a roof install you would start with all the water going straight to the pool and then turn the valve to send more and more water to the panels, stopping when the panels fully prime and the vacuum breaker valve closes (if you have one).