How these thermostat work is they use a thermostatic bimetal spring and as its heated it looses its tension.
In turn the oil pressure pushes the valve open allowing the oil to pass through as the springs cools it stiffens and it closes or restricts the oil flow.
Its a very very low cost and dependable old design but not very accurate but it works.
Here are some examples
https://www.alibaba.com/countrysearc...l-springs.html
https://www.alibaba.com/product-deta...2bd13975OYV1z8