Altars are core to domination mode, objects you conquer to generate points.
Blah, blah, blah
No need to explain what happens inside. 

All you need to know is: every altars is armor_stand with tags "immune" and "Altars".
You can just spawn entity with those tags, but thats not all.
You have to give it CustomName, this will be displayed in messages about conquering during the game.
After that, you need to use 'function altars:setup'
Finally, fully-functional altar will appear.

There is example of command to summon altar. Try it yourself.
If I remember well, there is a gimmick with coords.
If you stand on the surface and you want altar to appear beneath you, use Y-2 or something like that,
summon armor_stand X Y Z {CustomName:"{\"text\":\"Yellow Side\"}",Invisible:1b,Invulnerable:1b,PersistenceRequired:1b,NoGravity:1b,Tags:["immune","Altars"]}