This blog has an excellent introduction to German archives: German Genealogy Collective
How to use the online website of the State Archives of Baden=Wuerttemberg: Lind Street Blog
Online video showing how to use the search function on the website of the State Archives of Baden-Wuerttemberg: Video in German
Geheimes Staatsarchiv: http://www.gsta.spk-berlin.de/ This Berlin archive has some online records.
Das Bundesarchiv: https://www.youtube.com/watch?v=HwjRuw8GBFc
Genealogical sources in the Federal Archive of Germany
Nachlassdatenbank: https://www.bundesarchiv.de/nachlassdatenbank/index.php?sid=6b6fcd2565b00b6893573
Tagebucharchiv.de Emmendingen. German Diary Archiv
For example, in this archive you will find 42 diaries of Emil Schmeider (1828-1895), a hardware merchant and drawing teacher born in Berlin. His writings and drawings from 50 years bear witness to the Baden Revolution, the German and Franco-Prussian Wars, the cholera pandemic, the Year of the Three Emperors, the solar eclipse, and much more
At 17, in 1845, he writes: "...because I have found no one to whom I could truly and completely surrender myself, and because I also wish to vividly recall these youthful struggles later on, I want to creForate and cultivate a friend for myself who will keep everything I tell him unchanged and always be ready to faithfully return it to me. And this friend shall be you, my diary. I will keep nothing hidden from you that arises within me and around me; my joys and my sorrows, my hopes and my wishes. All this I will entrust to your secret pages, and chatting with you shall be dearest to me."
Inteview about the archive: https://tagebucharchiv.de/daad-cambridge/
Museum in the German Diary Archive, Marktplatz 1, 79312 Emmendingen, Germany, Tel. +49 7641/574659, dta@tagebucharchiv.de, https://tagebucharchiv.de