Plattdeutsch is considered a full language rather than a dialect. It overlaps a lot with Hochdeutsch, but is just as close to Dutch and share some grammar and a lot of vocabulary with English.

Here is a speech in the German parliament which starts in Hochdeutsch and then switches to Plattdeutsch https://youtu.be/sNWZq49srH8

There are entire books about Hochdeutsch and various dialects. Plattdeutsch is one particular dialect which has similarities to the Frisian language. The differences are to many to write about here. Best is to listen to a podcast or video in Platt