The best protection against theft is provided by sturdy chain, U-locks or folding locks that are large enough to lock the bike to a fixed object. Thin chain, U-locks or cable locks, on the other hand, are easy to pick with simple aids or tools. Spoke/frame locks are also not sufficient protection against theft. You should only use them to secure your parked bike if you can keep an eye on it.
When buying, pay attention to quality and stability.
The lighter and more expensive the bike, the heavier and more expensive the lock should be.
You can find help in your search for a good lock at:
www.vds-home.de in the "Burglary & theft" section under the keyword "Bicycle locks" 
www.adfc.de and at Stiftung Warentest at www.test.de

Always lock the bike frame to a fixed bike stand or another firmly anchored object. Locking only the front and rear wheels is not enough to prevent theft. The bikes can easily be carried away or loaded. 
Always secure your bike with a lock, even in closed garages or cellars, and make sure that access doors are locked. 
Always take valuable accessories such as the battery with you or secure the battery with an additional lock. Without the battery, the bike is practically worthless.
Because of the high value of electric bikes, you should generally use several locks to prevent theft.

Further protection against theft is provided by attaching a hidden transmitter to the bike. These GPS trackers alert the owner by text message if the parked bike moves and constantly transmit the current location.
Always inform the police and never set off on a search on your own.

The BikeManager app from bike insurer Wertgarantie provides digital registration for iPhones and Android smartphones.
Unlimited bike passes for all bikes, e-bikes and pedelecs can be created and managed free of charge in BikeManager. In addition, bikes can be registered free of charge on the website www.fahrradpass.info.