Website / Shop Access Restricted To Registered Users

[kempermusic]

I am planning to create a multisite Magento store that may only be accessible to B2B customers.

• The users and user groups should be created via the backend, a registration via the frontend is not planned.
• Depending on the user group, the users should be redirected to a website assigned to the user group after successful login.
• Access to the Magento websites, on which products and prices are displayed, may only be visible to logged-in users.

Is this realizable with Magento 2.4 by default -and if so: how- or is an extension required?

[toniagency82ed]

In Magento 2 open source, it is not possible out of the box. 

In Adobe Commerce it is doable out of the box utilising category permissions and shared catalogs. The entire B2B module of Adobe Commerce is probably something you'll want to use for this client.