Germany does not include a national index to their records. You need to know the town or city they lived and usually the parish they attended. Parish churches could cover serveral nearby towns. The parish where your ancestors’ records are located might be different from where they actually lived.
Also, shifting boundaries over the course of history affect your research as well. Poland/Germany and France/Germany boundaries shifted over time as an example.
These links may help in your research.
3 Essential Websites for German Family History Research -Legacy Tree
Compgen.de – German Genealogy
Landesarchiv – Baden-Württemberg
Bremen Passenger Lists
German Script Tutorial
Many Roads – German Immigrants 1850’s to 1890’s
Open Library – A collection of over thirty thousand names of German, Swiss, Dutch, French and other immigrants in Pennsylvania from 1727-1776
German Emigration Records
About Bremen in the world
Emigration from Southwest Germany (Click English on right toolbar)
Bremen Passenger Lists (flag above right nav is option for English or Deutsch).
Archives Bas-Rhin France – Click on box I accept these conditions, then select one of the conditions. You can try both options to see what you like best. The second one is the one I used for finding my Untereiner family with the help of a Alsace-Lorraine Facebook member.
Archives Haut-Rhin France – I use google translate with this one.
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