Is there a way to make a category that will show in all categories in wordpress?

For example, my categories in my website:

• Cat A
• Cat B
  • Cat B1
  • Cat B2
  • Cat B3

I want post tagged in Category A will be showed also in Category B and its children. So when they check mywebsite.com/category/cat-b1 they will also find Cat A.

I do not really know how to code, but I always can follow steps. Thank you a lot for your help.