how Can I set up a Raspberry Pi 4b to work as access point with dhcp, broadcasting its own network while simultanously connected as client to another access point. Devices connected to RPi's AP should not be visible outside its network. RPi should be accessible from both networks. https://www.raspberrypi.org/documentation/configuration/wireless/access-point-routed.md I am looking for that configuration, but fully wirelessly.
You can configure the Raspberry Pi as access point together with a client uplink to a remote hotspot if you define an additional virtual interface for the WiFi, e.g. named ap0 (or what you like). Then you can use wlan0 as usual to connect as client to the remote hotspot and use ap0 to use it as access point. Then its only a routing or firewall issue to protect subnets. In your case it is easy to have the devices, connected to the access point of the RasPi, invisible to outside networks. The use of Network Address Translation (NAT) ensures this. This also ensures that you can connect to the RasPi itself from all subnets. For an example how to do it you can look at Access point as WiFi router/repeater, optional with bridge.