htaccess - RewriteRule without redirect not working

You can add a rewrite rule to map to one of those pages with something like this:

add_action( 'init',  function() {
    add_rewrite_rule( 'myparamname/([a-z0-9-]+)[/]?$', 'index.php?myparamname=$matches[1]', 'top' );
} );

The first parameter of add_rewrite_rule is a regular expression to match a URL. The second parameter is an index.php non-pretty permalink URL, the URL parameters map directly into WP_Query arguments.

So if we take your first page as an example collections/collection/example:

add_action( 'init',  function() {
    add_rewrite_rule( 'collections/collection/([a-z0-9-]+)[/]?$', 'index.php?name=collection&collection_parameter=$matches[1]', 'top' );
} );

name=collection tells WordPress we want the page with the slug collection, and collection_parameter is how we’re passing the final parts value.

We then need to allow the use of collection_parameter, and adjust the page template to use it instead of $_GET

To grab the value from your page template:

$collection = get_query_var( 'collection_parameter' );

To whitelist the parameter we need to add it to the whitelist:

/**
 * Register custom query vars
 *
 * @param array $vars The array of available query variables
 * 
 * @link https://codex.wordpress.org/Plugin_API/Filter_Reference/query_vars
 */
function myplugin_register_query_vars( $vars ) {
    $vars[] = 'collection_parameter';
    return $vars;
}
add_filter( 'query_vars', 'myplugin_register_query_vars' );

Don’t forget to flush rewrite rules, and you can extend this for the other two pages.

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