Need help customizing the forms…halp!

To change things with the login form inputs (i.e. putting in placeholders), you can use the wpmem_login_form_rows filter:
https://rocketgeek.com/plugins/wp-members/docs/filter-hooks/wpmem_login_form_rows/

There is a ready-made code snippet (and some for the register form and widget login as well) in a tutorial post available to support site subscribers:
https://rocketgeek.com/filter-hooks/replace-form-label-tags-with-input-tag-placeholders/

Upload fields are kind of browser specific. But you can generally customize them with CSS. Try a google search:
https://www.google.com/search?q=css+for+file+upload

For more complex CSS, you may want to implement a custom stylesheet for the plugin, but for most instances, the plugin does allow for you to work on form elements in the WordPress Customizer so you can use the Customizer’s Custom CSS feature, which is fairly easy to do. There are instructions in the plugin’s docs here:
https://rocketgeek.com/plugins/wp-members/docs/customizing-forms/using-the-wordpress-customizer/

You can customize the Heading and Submit button text for the login form with the wpmem_inc_login_args filter:
https://rocketgeek.com/plugins/wp-members/docs/filter-hooks/wpmem_inc_login_args/