For some plots of land, it does make sense to have a construction company and some favorite houses in mind beforehand, especially if the plots have difficulties and the construction company can check whether the ideas are feasible. Or not?
Well... My opinion: conditionally.
Basically, you should already familiarize yourself a bit with the regulations, guidelines, etc., not only for house building but especially there, so that, for example, you can roughly read the development plan.
Then you should know your space program, how much space for what and roughly where. What can you live with, what not, what is mandatory or never to have. How much money is available a) for the plot of land, b) for the house construction. What do the ancillary building costs include in Hamburg or the surrounding area. Basement yes or no... Hamburg -> flooding, bomb disposal or search.
A construction company will probably send you home again if you want something from them without having a plot: it is a waste of their time to deal with everyone who might never find a plot that is free to build on, i.e., without a developer.
Or they make you sign something that binds you to them and can be expensive for you.
Get information from several and gain your experience - you can ask concrete questions gladly, you are more likely to get an answer than with a question like "where can I find the best car?" 19 people, 19 opinions