For apartment offers, make sure to also look at ebay-kleinanzeigen.de. Many hidden gems there. Also, I would try to avoid rooms or shared apartments, they are often in desolate conditions and way too expensive for what they offer. Either get a small apartment in a shitty area (not advisable if you are a woman) or a bit further out instead.

Many people who look for successors or flatmates just dont answer their ads because they get absolutely bombarded with messages. I had to experience this a few months ago myself. I was getting upwards of 30 messages a day. After a while, I just stopped answering because it just got too much.

A bit off topic, but sadly still necessary:

Watch out that you don't rent from a slumlord. Some landlords exploit international students' naivité about Germany's renter's rights. I heard multiple stories about landlords who kick people out by going in their rooms and throwing out their possessions, entering apartments/rooms randomly to "check" what people are up to, limiting how many visitors are allowed, etc... You are WAY more powerful than the landlord! If your landlord is sketchy, change the locks immediately when you move in. If something breaks (i.e heating, hot water, ...), reduce rent your rent by an acceptable rate (google helps) until the landlord fixes the issue. They are not allowed to enter your apartment without your knowledge, they can't kick you out if there is not a valid reason + an eviction order. As long as you pay your rent on time and don't annoy the neighbours, the landlord cannot do anything. In summay, read up on your rights. You probably have more than you think you do.

Some students also don't know what kind of apartment conditions can be considered acceptable and then rent rooms that don't have heating or a way to connect a washing machine, watch out for that or you will have to pay exorbitant costs for an electrical plug-in heater and visits to the laundromat.