If you want to alter html, generally you'll be working with hook_preprocess_hook or a twig template override. This is the case for views as well. In the case of views, the template that renders the title is views-view.html.twig
.
So if you wanted to render the title with a b tag, you could do:
function my_module_preprocess_views_view(&$variables) {
$existing_title = $variables['title'];
$variables['title'] = [
'#type' => 'html_tag',
'#tag' => 'b',
'#value' => $existing_title
];
}
However, it's frowned upon to use tags for presentation and preferred to use classes+css. If you just want to add a class to target, you could do a similar thing:
function my_module_preprocess_views_view(&$variables) {
$existing_title = $variables['title'];
$variables['title'] = [
'#type' => 'container',
'#attributes' => [
'class' => [
'views-title', // or whatever
],
],
'#value' => $existing_title
];
}
Both #container
and #html_tag
are render elements, and there are a lot available to you.
Of course, depending on what your theme and other modules are doing, the existing $variables['title']
might not just be a string, so you may need to adjust accordingly.