A very quick overview on what you need to add to add a block region in a Moodle theme template.

Config.php

Create a region id name then add it to your ‘regions’ paramater array for the page template (or templates) you want to use that block region on. You’ll need to set a defaultregion if you have a set region.

$THEME->layouts = [
    'base' => array(
        'file' => 'columns2.php',
        'regions' => array(),
    ),
    'frontpage' => array(
        'file' => 'frontpage.php',
        'regions' => array('side-pre','side-top'),
        'defaultregion' => 'side-pre',
    ),
];

/layout/frontpage.php

You will want to add the following lines of code to this file (or the template file you are adding the region to).

$blockshtml = $OUTPUT->blocks('side-pre');
$topblockshtml = $OUTPUT->blocks('side-top');

$hasblocks = strpos($blockshtml, 'data-block=') !== false;
$hastopblocks = strpos($topblockshtml, 'data-block=') !== false;

In your template context you’ll want to add the following parameters

$templatecontext = [
    ...
    'sidepreblocks' => $blockshtml,
    'sidetopblocks' => $topblockshtml,
    'hasblocks' => $hasblocks,
    'hastopblocks' => $hastopblocks,
    ...
];

/template/frontpage.mustache

Then in your template mustache file you will want to add the following to display any blocks in that region on your page.

{{#hastopblocks}}
	<aside data-region="blocks-column" class="d-print-none topblocks py-1 px-2" aria-label="{{#str}}blocks{{/str}}">
		{{{ sidetopblocks }}}
	</aside>
{{/hastopblocks}}

/lang/en/theme_name.php

You’ll also want to add a language string for any new block regions you add.

$string['region-side-top'] = 'Top Region';