is this plugin htaccess similiar to 301 redirects?

Hi, they are very similar I would think. However you might like to read more about 301, 302 and https ranking from the following URL 301-redirection-rules-for-seo.

The plugin page says:
“Force Load Static Files Using HTTPS”

which I think means force load static https://www.mydomain.com/ links in the post/page

But what bout the: https://www.mydomain.com/js/javascript.js in the <header> will that be forced loaded too?

Is this force loading static files temporary? should I bulk replace the static https://www.mydomain.com/&#8230; with https://www.mydomain.com/&#8230; in wp_post table later?

Static content is referring to the images, URL links embedded in the site etc. Do you also have the following feature enabled Automatic redirection to the “HTTPS”.?

This is forcing static file permanently while the plugin is active via the code added in the .htaccess file. However you should check to make sure all your files are now set up as https and not http.