Not strange at all. You’re on Multisite, that’s why I asked what URL ??
Plugins
* Go to https://example.com/wp-admin/network/plugins.php – Are they listed there? Note: DO NOT network activate them. I’m asking you a yes/no question here, NOT giving you directions yet.
* If the plugins ARE listed there, go to https://example.com/wp-admin/network/settings.php – Scroll down to “Menu Settings” and see if the box to enable plugins is checked. IF NOT that’s why the individual sites can’t see plugins. Check that and you will allow users to activate plugins per site.
ONLY network activate plugins that absolutely, 100%, MUST be active on all sites. DO NOT EVER activate all your plugins network wide without thinking about it first. It’s serious.
Themes
If you want a theme to be available on ALL sites in a network, go to https://example.com/wp-admin/network/themes.php and click on Network Enable
If you want a theme to be available ONLY on a specific site, it’s a little convoluted. Go to https://example.com/wp-admin/network/sites.php and hover over a site. Click on EDIT to edit that site’s settings.
On THAT page, click on THEMES and enable the themes you want.