Why does the site open old files from the server?

Question

caching

Previously, there was a problem that the site did not load css and js files if they were updated.
Now they are checked for time and it seems that they began to load correctly, but this is on a PC.
On the phone, when I logged in, for some reason, not only the old css is loaded, but also the old php files with different markup and html code.
Although, when I first went to the site from the phone after the update, everything seemed to be updated, but subsequent visits for some reason open the old version to me.
How to solve this problem on the server?