Change Defaul page to Categories

You could add a custom query to your theme’s index.php template file.

in this link there are many codes and they confuse me please can you give me the specific code that it will do what i want

So do you want to link them to a random category? Or just a category of your choice?

You could set up a redirect from your index.php, or turn your index.php into a custom category page itself.

If you simply want to redirect, which does slow down site loading time, then put in

header("location: https://www.site.com/?cat=7");