Well - ubuntu is the image server, not the OS... as counter intuitive as it is.
lxc remote list will give you a list of enabled remote servers
+-----------------+------------------------------------------+---------------+-------------+--------+--------+
| NAME | URL | PROTOCOL | AUTH TYPE | PUBLIC | STATIC |
+-----------------+------------------------------------------+---------------+-------------+--------+--------+
| images | https://images.linuxcontainers.org | simplestreams | none | YES | NO |
+-----------------+------------------------------------------+---------------+-------------+--------+--------+
| local (default) | unix:// | lxd | file access | NO | YES |
+-----------------+------------------------------------------+---------------+-------------+--------+--------+
| ubuntu | https://cloud-images.ubuntu.com/releases | simplestreams | none | YES | YES |
+-----------------+------------------------------------------+---------------+-------------+--------+--------+
| ubuntu-daily | https://cloud-images.ubuntu.com/daily | simplestreams | none | YES | YES |
+-----------------+------------------------------------------+---------------+-------------+--------+--------+
Most guides assume you're using the images remote server
You'd need the images remote server - most guides assume that you are using the ubuntu remote server - hence the "ubuntu:version" syntax
You can search for centos images, in the images remote with lxc image list images:centos
This will give you the listing of everything that's centos - something like
+----------------------------------+--------------+--------+------------------------------------------+--------------+-----------------+----------+-------------------------------+
| ALIAS | FINGERPRINT | PUBLIC | DESCRIPTION | ARCHITECTURE | TYPE | SIZE | UPLOAD DATE |
+----------------------------------+--------------+--------+------------------------------------------+--------------+-----------------+----------+-------------------------------+
| centos/6 (3 more) | fa76b2602447 | yes | Centos 6 amd64 (20201119_07:08) | x86_64 | CONTAINER | 75.79MB | Nov 19, 2020 at 12:00am (UTC) |
+----------------------------------+--------------+--------+------------------------------------------+--------------+-----------------+----------+-------------------------------+
| centos/6/cloud (1 more) | 606ddaf55044 | yes | Centos 6 amd64 (20201119_07:08) | x86_64 | CONTAINER | 84.11MB | Nov 19, 2020 at 12:00am (UTC) |
+----------------------------------+--------------+--------+------------------------------------------+--------------+-----------------+----------+-------------------------------+
| centos/6/cloud/i386 | 999be8e85790 | yes | Centos 6 i386 (20201119_07:08) | i686 | CONTAINER | 84.25MB | Nov 19, 2020 at 12:00am (UTC) |
+----------------------------------+--------------+--------+------------------------------------------+--------------+-----------------+----------+-------------------------------+
| centos/6/i386 (1 more) | 543d3d8c6640 | yes | Centos 6 i386 (20201119_07:08) | i686 | CONTAINER | 75.93MB | Nov 19, 2020 at 12:00am (UTC) |
Lets say I wanted a plain vanilla centos/6 AMD64 container
lxc init images:centos/6 test would work.
You can then start up and work on your centos container.
I found this page pretty helpful in working out and understanding the commands, though I had to do a little poking around to find out where centos was