There are two basic designs (with some variations) -- "blocking" and "bypass."
Many gas engines are "blocking". The thermostat regulates flow to the radiator, and the water pump "dead heads" (ex any flow allowed to the heater core) when the thermostat is closed. If you look at this typical gas engine thermostat
https://shop.advanceautoparts.com/p...Vs41bCh1LVQtxEAQYAiABEgKvzfD_BwE&gclsrc=aw.ds you'll see it has only one "valve" in it -- at the top.
Most diesel engines are designed as a "bypass" configuration. When the thermostat is closed coolant circulates in the engine but does not go through the radiator. When it is fully open the bypass passage is fully blocked (and the radiator passage is fully open.) There are also some diesel designs that have TWO thermostats (one for each bank in a "V" engine); 2-stroke Detroit "V" engines are typically set up this way.
If you look at this thermostat for the TDI engine
https://www.autohausaz.com/pn/13090...MIv-6f9oXd3gIVBI9bCh2WgwLjEAQYBSABEgKK0PD_BwE you will see that it has a bottom plate along with the top coolant passage and
two springs. When the thermostat is closed this design forces coolant through the bypass. When the thermostat begins to open the plate closes off the bypass passage and forces the coolant through the radiator. A blocking thermostat does not have the bottom plate, second spring or the bypass passage in the block at all.
In both cases the thermostat is usually partially open in normal operation. That is, SOME coolant goes through the radiator, but not all -- the rest is either bypassed for a diesel or simply not circulated at all in a gas engine. When the thermostat is fully open the system is operating at its maximum heat-rejection capacity and if more rejection of heat is required coolant temperature will rise and, unless the additional requirement is modest and can be met by the increase delta in temperature between ambient air and the coolant temperature, it will overheat.
(There are plenty of exceptions to the rule that gas engines are "blocking"; in particular I know the SkyActiv Mazda engines use a bypass configuration, and I'm sure there are others -- most-likely newer engines -- that do as well.)