They were defined by law. They are supposed to have roughly equal populations and must be re-assigned if the difference in population exceeds 25% (I don't know what that means in practice, e.g., what the reference is.)

Here's a list: https://en.wikipedia.org/wiki/List_of_Bundestag_constituenci...

They start "re-districting" if a district deviates from the average population by ±15%.

The new districts are proposed by an independent commission and have to be approved by parliament. Here is the proposal for 2021 (pdf warning): https://www.bundeswahlleiter.de/dam/jcr/b1752d5e-aecd-4cff-9...

And here is the decision by parliament, I guess: https://dip.bundestag.de/vorgang/.../261890