HI Abdussamad,
The step is to follow best practice where only CMS, IFRS, OFRS and tomcat comes into picture when you do a patching or update. Check :KBA-1757132 - BI4 Support Pack Upgrade and Patch Install Best Practice Guide
So basically, you need to disable all servers and stop them and only CMS, IFRS, OFRS would be up and running.
Yes, when performing the update it would be downtime for the users and no one should connect to it.
Regards,
Mahak