A few month ago arrived on BA and departed on LH about 3 to 4 hours later. There is a transit sign and a waiting area. When you get there they take your passport, ticket and make you wait. Around 2 hours later somebody whowed up with all and the boarding passes and we were allowed to go upstairs to the departure level without passing immigration.

Everybody was nice and friendly there, just a little scary to be without passport etc.

Only carry on that time, so no experience with luggage.
If i had more than 3 or 4 hours i would go thru immigration and sit in a restaurant instead of waiting in the transit area.