When using https, WordPress doesn’t use https for CSS and JavaScript, and admin doesn’t work. How do I fix this?

Check two things:

• in the wp-options table, the site URLs (in two places) should be the full URL, as in https://www.example.com .
• check the site’s htaccess file for proper rewrite of http to https.

For a htaccess rule, this one works in most case:

RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

Then look at your theme files to ensure that any references to CSS/JS include the https protocol, not http. Most well-written themes will use relative rather than absolute references to files to include.

It’s also possible a plugin might be specifying http rather than https.

Use the Network tab in Developer Tools (usually F12 in your browser) to look at each request to see where it is coming from. If the request is to a plugin folder, then check the code in there.