Modify customizr\inc\parts\class-header-header_main.php

See also How to customize Customizr.

For the footer credits, see the credits snippet.

I am not in a position to learn new programming languages of hooks, API, filter, action…whatever, this can take days and I don’t have even a minute to waste.

Well, your question was specifically “How can I modify this file customizr\inc\parts\class-header-header_main.php …”, so it looked like you did.

It sounds like you might need to hire someone to help you. Or change theme to a less-flexible, but easier one.

I was thinking of adding the google structured markup in an hour’s time or at max a day

If you’re talking about things like opening hours and addresses etc, you don’t need to edit that php file in any case. You can put things like these in the place in your site where you keep the information.

For example, in my widget areas, where I have the address/tel etc., I have something like the following for the address:

<div itemscope itemtype="http://schema.org/LocalBusiness">
	<p itemprop="address" itemscope itemtype="http://schema.org/PostalAddress">
		<span itemprop="streetAddress">Some Street</span><br />
		<span itemprop="addressLocality">Some city</span>&nbsp;
		<span itemprop="postalCode">012345</span>
		<span itemprop="addressCountry">Some Country</span>
	</p>
	<p>Tel: (+xx) <span itemprop="telephone">123 45678</span></p>
</div>

and something like the following for the opening hours:

<div itemscope itemtype="http://schema.org/LocalBusiness">
	<p itemprop="openingHoursSpecification" itemscope itemtype="http://schema.org/OpeningHoursSpecification">
		<meta itemprop="opens" content="09:00:00">
		<meta itemprop="closes" content="13:00:00">
		<meta itemprop="opens" content="14:00:00">
		<meta itemprop="closes" content="19:00:00">
		<span itemprop="dayOfWeek" itemscope itemtype="http://schema.org/DayOfWeek">
			<span itemprop="name">Mon&ndash;Sat:</span>
		</span>
	 9am&ndash;1pm&nbsp;&nbsp;2pm&ndash;7pm
	</p>
	<p>Saturday in summer:&nbsp;&nbsp;9am&ndash;1pm</p>
	<p>August:&nbsp;&nbsp;call&nbsp;to&nbsp;check&nbsp;if&nbsp;we're&nbsp;open</p>
</div>

The opening hours are over-ruled by the business’s entry in Google Business.