This Esri article doesn't explain how to become a root for the AWS Marketplace instance that runs Ubuntu. This guide is useful for post-installation steps.

Step by step guide

Use "ubuntu" username instead of "arcgis"

1. Open SSH port 22 in security group of the instance. By default, only HTTP/HTTPS ports are open.
2. ssh -i ~/.ssh/Joe-SDT.pem ubuntu@(IP address)
3. sudo -i
4. apt-get update
5. apt install emacs
6. apt upgrade
7. It's OK to apply security patches and upgrade packages

reboot

Note: "arcgis" user account has no su privilege

arcgisserver.service

Although the Esri guide says that the service file needs to be copied over, it is already installed.

root@ip-(IP address):~# diff /arcgis/server/framework//etc/scripts/arcgisserver.service /etc/systemd/system/arcgisserver.service 
4,25d3
< #
< # Configure ArcGIS Server to be started at boot on Linux distributions 
< # adopting systemd init system (For example RHEL 7.x and SuSE12) by 
< # following these instructions:
< #
< # 1.) Switch to the root user.
< #
< # 2.) Copy this file to /etc/systemd/system
< #
< # 3.) Enable the service to start at boot:
< #
< #     # systemctl enable arcgisserver.service
< #
< # 4.) Verify systemd service is setup correctly:
< #
< #    # systemctl stop arcgisserver.service
< #    # systemctl start arcgisserver.service
< #    # systemctl status arcgisserver.service
< #
< # 5.) Reboot the system and verify that Server restarts properly.
< #
< # ------------------------------------------------------------------
34,39d11
< GuessMainPID=false
< 
< # The minimum number of processes need to be set to 25059 or higher.  Enable
< # and raise this limit if it is a heavily used system.  Use ulimit -Su -Hu to
< # check current values.
< # LimitNPROC=25059
41,49c13,14
< 
< # To prevent any one service from spawning too many threads and consuming all
< # server resources, systemd v228 and beyond included in SLES12 SP2 and higher
< # set the maximum number of threads to be created at 512.  Users on SLES12 may
< # need to enable and raise this limit if it is a heavily used system.  Use
< # "systemctl show --property DefaultTasksMax" to check the current value.  To
< # find the version of systemd, use "systemctl --version".
< # TasksMax=512
< 
---
> LimitNPROC=25059
> GuessMainPID=false
51a17
> Environment="arcgis_cloud_platform=aws"
55d20
<  
However, it is not active.

root@ip-(IP address):~#  systemctl status arcgisserver.service
● arcgisserver.service - ArcGIS Server Service
  Loaded: loaded (/etc/systemd/system/arcgisserver.service; disabled; ven
  Active: inactive (dead) 
Enable it.

root@ip-(IP address):~# systemctl enable arcgisserver.service
Created symlink /etc/systemd/system/multi-user.target.wants/arcgisserver.service → /etc/systemd/system/arcgisserver.service.

References

1. https://enterprise.arcgis.com/en/server/latest/administer/linux/configuring-https-only-using-the-default-self-signed-certificate.htm
2. https://enterprise.arcgis.com/en/server/latest/administer/linux/configuring-https-using-a-self-signed-certificate.htm