Need help with testing and build system resources

I’m a newbie frontend developer. While I’m comfortable with developing applications, I’m struggling with 2 things.

1. Build Systems: I leverage create react app and it’s typescript template while developing applications. I understand how things work at a high level. I struggle with configuring things from scratch. Do you have any resources that would help create a mental model of how things work when it comes to build systems in general?

2. Testing: This is something I’m clueless about. I’m struggling with below aspects.

   a. How is testing different for FrontEnd?
   b. How does one test UI aspects of frontend vs the functionality?

Note: I’m new to Dev.to as well, please let me know if there is a different forum to ask for suggestions/recommendations.