Product page: https://www.aosong.com/en/products-60.html

Official Datasheet (EN): aht21_datasheet_a1_20210323.pdf (AOSONG)

AHT21_Datasheet_A2.pdf

Lib: https://github.com/Thinary/AHT_Sensor

AliExpress: ~2$

See some test results:

Test 3 v10a high to low humidity

Test 1 v10b humidity absorber

Test 5 v10b NaCl solution draft