I’m including a html file in my plugin, but it can’t find its css or js files

you must use enqueue script and style commands to include wp.

for it, look at these links:

https://developer.wordpress.org/reference/functions/wp_enqueue_script/

https://developer.wordpress.org/reference/functions/wp_enqueue_style/

also dont use hard php commands like this : <?php echo(__DIR__) ?>

there are wp commands for these jobs.

you can use below:

plugin_dir_path

plugins_url

plugin_dir_url

plugins_url