Yup, that's what Germany is, all over the place & LOTS of it. Pick an era or field & you've got a lifetime's worth of collecting.
I don't know of any specific German sites which will help what you're after but I can give you some pointers.
First, a catalogue of Germany is essential & needs to be studied. Scott's is good & has information on which stamps were overprinted & where to find them. If you're semi-serious them Michel German Specialised is the way to go. For the serious collector there are many specialised handbooks to keep you happy. (My Upper Silesia Handbook is 4½ inches thick!)
From what you've got, the top 2 pix are from East Germany (Deutsche Demokratische Republik), & are the later official reprints for stamp collectors which every schoolboy collection had in abundance & which are valueless. It's the position of the last "e" in "Deutsche" which is important
LEFT - Original Print. RIGHT - Philatelic Reprint
Anything with "Tausend", "Million" & "Milliarde" is from the Inflation period 1922-23
"Dienst", or what looks like "Dienft" is Official so BOB for Scotts
Finally, "Rumanien" on the German "Germania" type is from Romania, WWI. The Germania issues were extensively surcharged & overprinted during & after WWI.