Which LTE and 5G networks does a China smartphone support?

This can only be answered for the exact model. Supported LTE and 5G bands are listed in the specifications. Band 20 (800 MHz) can matter for coverage outside cities in Germany, but does not replace checking carrier bands and VoLTE requirements.

The Compatibility button checks stored mobile-network bands. It is a guide, not a guarantee of coverage, speed or every network feature.