Can Germany immigrate after going to school?

The nationality system in Germany is the same as that in Japan. Foreigners don't really have German nationality, but they have permanent residency.

After graduation, you can get a one-year visa according to the current residence law, and you can find a job related to your major within this year. If you have a job, you have to change your original visa into a work visa. From the time you get a new work visa, you can apply for permanent residence if you have lived and worked in Germany continuously and steadily for five years and intend to stay like this.

The conditions for permanent residence are that you have lived in Germany for more than five years (generally referring to long-term visa types such as work visas), have a stable income, have enough housing space, have no criminal record, and speak German.