[CentOS] Yum Update Question

I have asked myself over the years... just what happens during the 
update process? For instance, the latest updates included updates to 
php. I'm wondering if apache is restarted or reloaded to read the new 
php updates?

More broadly... if this is done, is it done pretty much for every 
interrelated update, I guess with the exception of kernel updates?

Thanks for all the hard work.

Best,
John Hinton