Microsoft Visual Studio Unit Test Doubts

I am using microsoft visual studio 2017 for my development. My project has lot of unit test cases written and working fine as well.

I have few doubts about unit test cases. It will be great if someone can answer my below questions.

1. Which are the available unit test frameworks in microsoft?
2. What is pex? Is it unit test framework.
3. Is there any design pattern for unit test? (i.e. mock unit)
4. Any example link for best unit test practice.

Overall, I want to redesign my unit test project which is in MSTest framework. Since i am developer, I don't have much knowledge about unit test.