I am currently using the Onepress theme and want to edit the header section, the part that prints the site title. However, I cannot for the love of life figure out which part actually controls the header. Header.php does not show anything and neither does the frontpage template.
In WP, you first need to understand the template hierarchy https://developer.wordpress.org/themes/basics/template-hierarchy/
So as per hierarchy,
-- home.php / custom page template.
If you have front-page.php , then please check which header file it is enqueuing.
If there is no front-page.php then check home.php exist in your theme file. If yes check which header file it is enqueuing .
If it doesn't have both file, goto Appearance >> Customise >> Static Front Page.
So if it has some template name in Front Page then you need to check in your template file for which header file it is adding.
For header file, if they are using get_header(), then it will use header.php from your active theme.
If it has get_header( 'home' ); then it will use header-home.php
So from there you can go customise your header.
Also if you are using child theme , then please make check if it has header related file in your child theme or not.