Around 80% of Warsaw was razed to the ground by the occupation forces before and then during the Warsaw Uprising. This included a campaign of door-to-door civilian executions.

The occupation also featured the destruction or plunder of all kinds of Polish art.

Wikipedia attributes the claim that German officials estimated "over 90%" of art previously in Poland was in their possession to Polityka III Rzeszy w okupowanej Polsce, Tom II (Politics of the Third Reich in Occupied Poland, Part Two) (in Polish); I cannot verify this as I sadly do not speak or read Polish.

One statue in particular torn down was of Adam Mickiewicz, a 19th Polish poet.

Poland endured attempted cultural extermination, and it is well documented and sourced.

The Wikipedia article is here: https://en.wikipedia.org/wiki/Polish_culture_during_World_War_II

Supplementing: https://en.wikipedia.org/wiki/Occupation_of_Poland_(1939%E2%80%931945)

See this for a particular event which highlights the despicable nature of both the Nazis and the Soviets: https://en.wikipedia.org/wiki/Warsaw_Uprising

Wikipedia is not a perfect source; it does, however, make for a quality source aggregate. This entire article is thoroughly sourced by proper writings and the sorts. Others that have been more objectionable tend to be about modern figures with limited to minimal or low quality sourcing; a chief example of which is the Project Veritas article citing mainstream news as to why Project Veritas is alt-right.

Anyone can list sources. Hell, anyone can write a book. At the end of the day there is only so much you can do to source information; having a wide variety of high quality writers with direct sources goes a long way. If you want to dismiss this information, you should go through each claim that you disagree with and it's sources to rebut it, or find stronger sources that say things to the contrary.