Early in the 1920s the new polish state was expansionistic and sort to incorporate areas with significant polish populations or populations they saw as “polonizable” that they saw as ethnic similars who could be assimilated.

So you saw the Polish-Soviet war where Poland took advantage of the Russian civil war to take parts of the USSR (which are today part of Ukraine and Belorusia I believe) and another campaign north seeking to annex Lithuania.

Early in the 1920s it was a vibrant multiparty democracy with essentially a left / right divide between worker and peasant parties on one side and a broad right wing coalition of industrialists, nationalists, and the major land owners on the other side.

Then in the mid-20s there was a de facto coup which saw a strong man from the right installed in power. It wasn’t openly a dictatorship, opposition parties were allowed to exist, but political power was firmly under the hands of Piłsudski up until the Nazi invasion.

Piłsudski was a right wing nationalist who wanted to continue incorporating territories it saw as rightfully Poland into Poland, for example during the partition of Czechoslovakia a part was annexed to Poland. He also pursued a campaign of assimilation towards “polonizable” minorities (eg Lithuanians, Ukrainians, Belorussians). Jews were also a target with various official discriminations against them, and later in the late 30s there was even a policy of removing Jews from Poland via a mix of deportations and encouraging immigration by restricting social services.

The other major minority in Poland was Germans. Germans did not actually face significant discrimination because they were clustered in the wealthy and industrialized part of Poland. There were some minor discriminations with the public service preferring Polish to some degree but they weren’t subjected to the same assimilationist policies as were “polonizable” Slavic populations in the east. So when Hitler was complaining about the harsh treatment of Germans he was in fact mostly making it up, but he was able to point to the treatment of Ukrainians and say “they want to do this to Germans.”