He talks about a specific overseas network (Mobistar) be the process is the same for ALL networks when using a Android Phone
On a cordless or modern land line phone
On a older land line phone with few or no buttons
Watch video on this: https://youtu.be/I6vBP04Q_gg
Even though conference calls on a mobile phone are limited to 6 people, the originator and 5 others and on landlines to 3 people, the originator and 2 others, in practice you can have an unlimited number of people on the one phone conversation.
This is because the non originators of the call, can each , in turn, act as originators and so call at least 2 other people, who inturn can call 2 other people, and so it goes on.