Here are a few articles explaining the Anglican tradition of Evensong:
Here's an article on Evensong from Classic FM: https://www.classicfm.com/discover-music/what-is-evensong-how-long-service/
The Religious News Service: https://religionnews.com/2017/08/30/evensong-sees-a-surge-even-as-british-church-attendance-declines/
From choralevensong.org/uk: https://www.choralevensong.org/uk/about-choral-evensong-724.php
For some reason, you have to go to "Read More" to use the links.
It helps to differentiate between "Evensong" proper, which is the Evening Prayer service sung by officiants and congregation, and "Choral Evensong", which is the Evening Prayer mostly sung by a choir.