Debugging WordPress with Xdebug - PHP Fatal Error: Uncaught Error: Call to undefined function get_header() - phpstorm

Question

I'm trying to debug a WordPress-installation using Xdebug and PhpStorm.
If I write this in the top of functions.php:
[line 1]: <?php
[line 2]: $test = '1234';
And set a breakpoint on line 2, and run 'Debug functions.php', then I hit that breakpoint and I'm able to debug that breakpoint in PhpStorm. So far so good!
But if I move it down, to the bottom of functions.php, and do the same thing, then I get the error:
PHP Fatal error: Uncaught Error: Call to undefined function add_action() in /path/to/project/wp-content/themes/my-theme-name/functions.php:44
#0 {main}
Stack trace:
thrown in /path/to/project/wp-content/themes/my-theme-name/functions.php on line 44
Line 44 looks like this:
[line 43]: include_once( __DIR__ . '/classes/AdminNotice.php' );
[line 44]: add_action('admin_notices', [new AdminNotice(), 'displayAdminNotice']);
I've also tried debugging other files.
Like page.php. Same thing - if I add my demo-code (and breakpoint) in the top, then it works.
But if I move it to the bottom, then it stumbles over get_header(). The error:
PHP Fatal error: Uncaught Error: Call to undefined function get_header() in /path/to/project/wp-content/themes/my-theme-name/page.php:6
Stack trace:
#0 {main}
thrown in /Users/zethodderskov/Code/Greenmind/V2-1__local/app/public/wp-content/themes/my-theme-name/page.php on line 6
And line 6 looks like this:
get_header(); ?>
And if I set a breakpoint in an imported class, then it doesn't stop at that breakpoint (even though I'm 100% sure that it reaches that part of the code).
This may be because it's reached during a POST-request. Hmm... I hate being a noob.
Useful intel
I've tried a bunch of times setting Xdebug up, but I've never been successful. This is the closest I've gotten. So it might be some stupid blunder.
I'm using PHP v. 8.0.6, Xdebug version 3.0.4 and PhpStorm version 2020.3.1 - Build #PS-203.6682.180.
The remote server is a Cloudways-server.
I've defined WordPress (and WooCommerce) as external libraries (Languages & Frameworks >> PHP). So PhpStorm should know the WordPress-functions. And I can CMD+click on the WordPress-functions and see the function definitions.
If I go to 'Run' >> 'Web Server Debug Validation' and click 'Validate', then it shows all green ticks.
The guide I've followed the most is this Zero-configuration Xdebug debugging.
Here is the contents of my local php.ini-file:
...
... Omitted lines
...
[xdebug]
zend_extension="/usr/local/lib/php/pecl/20200930/xdebug.so"
xdebug.mode=debug
xdebug.client_host=12.123.123.123 (IP to Cloudways-server)
xdebug.client_port=9003
; End of file
In Cloudways, I've added these lines to the php.ini on the remote server (under 'Application Settings' >> 'PHP FPM Settings'):
php_admin_value[display_errors]=on
php_admin_value[memory_limit]=2048M
php_value[xdebug.remote_enable]=1
php_value[xdebug.mode]=debug
php_value[xdebug.client_host]=87.116.38.66
Answer to comments 1
As #LazyOne pointed out, then I might be debugging a single file, whereas I'm trying to debug the entire project running.
I'm just unsure how to do that. If I go to 'Run' >> 'Edit configurations' (under debug), then I reach this dialog box:
And I'm not really sure what I'm supposed to do here, since it 'forces me' to define which file I'm trying to debug.