Re: Skype for Business
"even though Skype for Business might nevertheless come with Office 362.5, it apparently cannot communicate with ordinary people using ordinary Skype with ordinary Skype accounts"
In an on-prem deployment, as long as Skype is federated and not restricted to block unknown domains, you can communicate with ordinary Skype accounts. (https://docs.microsoft.com/en-us/skypeforbusiness/set-up-skype-for-business-online/let-skype-for-business-users-add-skype-contacts)
For Office365 Skype accounts, it also requires federation to be enabled although there appear to be some restrictions for Germany and a specific provider. (https://docs.microsoft.com/en-us/skypeforbusiness/set-up-skype-for-business-online/allow-users-to-contact-external-skype-for-business-users)
Other than that, it's likely to be a security policy issue or a configuration issue preventing the integration. Unless something has changed since 2018...