In Poland since 1918, it's been the case that the citizenship could only be earned by descendants of Polish citizens.
However, a big one-time change – which turned "almost everyone" into a citizen – came through the Polish Citizenship Act of 1920, Article 2 and 2a. On January 20th, 1920, everyone became a Polish citizen who was a resident on the new, greater Polish territory and had no other citizenship.
To be a resident on the new, greater Polish territory, one had to prove it either by papers showing his citizenship in the former Kingdom of Poland; or Prussian, Russian, or Austrian/Hungarian documents showing the residence with a specific proof that he or she lived on the territories later acquired by the new Polish state.
The citizenship was also given to some exceptions based on international treaties; and, after a 1938 update of the law, most of the former citizens of Czechoslovakia in the vicinity of Těšín that was partly conquered by Czechoslovakia after the (tied) 1919 Czechoslovak-Polish seven-day war, and re-acquired by Poland as a gift from Adolf Hitler in 1938 (no one had to ask Czechoslovakia since the Munich Treaty).
Many people were of Polish origin but hadn't lived on the "right" territory before 1918 so they didn't become Polish citizens but were recognized as a special category of people of "Polish origins". In practice, this was the first step to becoming a Polish citizen if (as Article 3 of the 1920 law says) they renounced the previous citizenship and declared their desire to become Polish citizens. Any "break" in the citizenship could have eliminated the citizenship for the family, and so on, see Polish nationality law.