As is the case with many things moodle, this goes deeper than meets the eye! 
Lots to look over/read/ and try to comprehend (I have a headache now! :|):
https://docs.nginx.com/nginx/admin-guide/content-cache/content-caching/
Section: Purging Content From The Cache in above link.
Section: Purging Content From The Cache in above link.
https://httpd.apache.org/docs/2.4/caching.html
https://www.php-cache.com/en/latest/
https://docs.moodle.org/403/en/Caching
https://docs.moodle.org/403/en/MUC_FAQ
https://docs.moodle.org/403/en/Redis_cache_store
https://docs.moodle.org/403/en/Performance_recommendations
and to top it off:
'SoS', Ken