Baseurl with hardcoded release version and with $releasever

I have some CentOS based VMs in Azure and recognized that depending on the precise base image I used to deploy them, a different repository configuration is in place. 1. Rolling Release Configuration

[base]
name=CentOS-$releasever - Base
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra 
baseurl=http://olcentgbl.trafficmanager.net/centos/$releasever/os/$basearch/ 
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

2. Fixed Release Configuration

[base]
name=CentOS-7.7.1908 - Base
#mirrorlist=http://mirrorlist.centos.org/?release=7.7.1908&arch=$basearch&repo=os&infra=$infra 
baseurl=http://olcentgbl.trafficmanager.net/centos/7.7.1908/os/$basearch/ 
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7

I have two questions: 1. What does the resulting repository configuration depend on? Is there also a way to install a "fixed release configuration" from an ISO image provided by CentOS via their website? I could not find one... 2. How to perform a release update with the "fixed release configuration" variant?