There are various sites that will tell you the theme and plugins used on any site. One is https://whatwpthemeisthat.com/ , there are many others.
In general, what you see when you login to the admin area depends on your user level. Admins see everything, guests see only their own profile. So it may be that your user is not an ‘admin’ level of privilege. Your question didn’t specify your user level.
It may also be that a plugin is restricting what you see. So you could use the above site (or similar) to see if there is a plugin installed that is limiting your access. You would have to look at the info on the plugins (via the plugin’s page) to see if there is something that is limiting your access.
A theme could limit what you see after logging in, but I would think that is less likely.
But, I suspect, given the limited information you provided, that you are not an admin-level user. So that would limit what you can see.
If you are using an admin-level account, then you will need to provide more info: your user privilege level, plugins installed, and theme used.