Sample theme template renderer for filter_units (https://github.com/frumbert/filter_units)

columns2.mustache

    {{>theme_mytheme/inc_start}}

    <div id="page" class="container-fluid">
        {{{ output.full_header }}}

        <div id="page-content" class="row">
            <div id="region-main-box" class="col-12">
                {{#hasregionmainsettingsmenu}}
                    <div id="region-main-settings-menu" class="d-print-none">
                        <div> {{{ output.region_main_settings_menu }}} </div>
                    </div>
                {{/hasregionmainsettingsmenu}}
                <section id="region-main">
                    <div class="card">
                        <div class="card-body">
                            {{#hasregionmainsettingsmenu}}
                                <div class="region_main_settings_menu_proxy"></div>
                            {{/hasregionmainsettingsmenu}}
                            {{#coursehome}}
                                {{> core/single_button }}
                            {{/coursehome}}
                            {{{ output.course_content_header }}}
                            {{{ output.main_content }}}
                            {{{ output.course_content_footer }}}
                        </div>
                    </div>

                    {{{ output.activity_navigation }}}
                </section>
            </div>
        </div>
    </div>

    {{>theme_mytheme/inc_end}}

columns2.php

$templatecontext = [
    'sitename' => format_string($SITE->shortname, true, ['context' => context_course::instance(SITEID), "escape" => false]),
    'output' => $OUTPUT,
    'sidepreblocks' => $blockshtml,
    ...
    'coursehome' => theme_mytheme_coursehome_button($OUTPUT)
];
echo $OUTPUT->render_from_template('theme_mytheme/columns2', $templatecontext);

lib.php

function theme_mytheme_coursehome_button($output, $consume = true) {
    global $SESSION;
    if (isset($SESSION->coursehome)) {
        $coursehomeurl = new \moodle_url($SESSION->coursehome);
        if ($consume) unset($SESSION->coursehome);
        $url = new \single_button($coursehomeurl, get_string('returntocourse', 'filter_units'), 'get', true);
        $data =$url->export_for_template($output);
        return $data; 
    }
    return null;
}