Modern web architectures use salable horizontal layers or clusters of servers:
Web Front End Tier
API Back End Tier
Database Query Tier
The instances in these horizontal clusters are all the same except for minor host differences.
Traditionally the instances would be hosts and would be long running instances. Updates and software releases would be installed individually on each host. Configuration management tools such as puppet and chef or home grown scripts would be used to manage the update process.
With virtual host based instances the cost of creating a new instance is low. Image cloning becomes very practical. The update process can be:
Create new base gold image and boot
Do a complete automated install from base OS of the server
Create a snapshot of image
Clone X copies of snapshot
Spin up new instance for each copy