Wordpress/CSS - Where Can I apply some media queries?

I am not familiar with customizing WP theme codes, I am still learning so I got couple of questions:

I want to apply some media queries on my site-branding h1.
In which .css document do I apply it? Does it even matter? Can I just take a random .css file and write the queries right away?

And another question:
Can I just make my own .css file and "link rel=" with the html file I want to style?

Thank u

You can make own .css and put a link to it at header.php or index.php in theme directory

Or you could be really lazy and install a custom css plugin.

Depends on the theme that you are using. If nothing is currently set to override whatever you'd be willing to change, changing it in the default CSS file or adding your own should work just fine.

I solved my issue.