This top section made with Centos 7.
GOTCHA!  For some reason with my latest install attempts with Centos 7, the User Data output
  is going to /var/log/messages instead of to /var/log/cloud-init-output.log.
See "Centos 9" below.

Default login user in the AMI is: centos

xfs FS is used.  How to extend?

AMIs with launch links:  https://centos.org/download/aws-images/

Default user 'centos'

Disable selinux.  Change to SELINUX=disabled in "/etc/selinux/config".

yum --enablerepo=extras install epel-release
yum -y install yum-plugin-priorities
Edit files /etc/yum.repos.d/*Base*.repo and epel.repo to set explicit "enabled=1"s and "priority=N"
    [base]: 20
    [updates]: 10
    [extras]: 30
    [epel]: enabled=0
yum clean all
yum -y update
timedatectl set-timezone America/New_York

yum -y install ntp
Edit /etc/ntp.conf.  Disable ^server lines and add: server 169.254.169.123
systemctl start ntpd.service
systemctl enable ntpd.service
service ntpd status

Enable epel in /etc/yum.repos.d/epel.repo
yum -y install exim nodejs npm p7zip nuttcp  speedtest-cli
Disable epel in /etc/yum.repos.d/epel.repo
yum remove postfix

# Locale already set to US UTF_8 by AMI
# No aws CLI installed
# TODO:  Determine if this is AWS CLI v2:
yum -y install awscli

# When edit /etc/hosts, leave the "localhost" lines as the are and append the other 2.

yum -y install... git + everything in ~/ec2/*bash with these modifications:
Unavailable in epel:  pkgfile  expac
Already installed:  cronie man-db dmidecode and from above: exim nodejs npm p7zip
    cronie is already conifigured and running (the job is labeled "crond" instead of "cronie").
'nc' instead of 'netcat'
'mailx' instead of 's-nail'
'java-11-openjdk-devel' instead of 'jdk11-openjdk'
'bind-utils' instead of 'bind'

yum install -y https://s3.<REGION>.amazonaws.com/amazon-ssm-<REGION>/latest/linux_amd64/amazon-ssm-agent.rpm
systemctl status amazon-ssm-agent

============================================
Centos 9

Default login user in the AMI is: ec2-user

Nicely, repos files now explicitly always state enabled=x for every repository.
By default only 2 repo files for centos and centos-addons, and they enable only these reposes:
baseos
appstream
extras-common

crb repository is for C and C++ dev and packaging frameworks like meson, dejagnu, doxygen.