Snapdragon 855 was indeed the chipset used in most flagship Android smartphones of 2019. I have one in my ASUS ZenFone.
Based on the big ARM cores that are included, it is easy to determine the manufacturing year of a flagship smartphone. The cheaper smartphones can continue to use older cores. Arm announces a core one year before it becomes used in smartphones.
Cortex-A72 => 2016
Cortex-A73 => 2017
Cortex-A75 => 2018
Cortex-A76 => 2019 (like in the tested Snapdragon 855)
Cortex-A77 => 2020
Cortex-X1 => 2021
Cortex-X2 => 2022
Cortex-X3 => 2023
Cortex-X4 => 2024