Ok, I see how the symbilic links works as the newest extetion files are used from the slave site. But what I don´t understand is that when I upgrade the master site, if there are sql upgrades, how are these updated in the slave sites?

In fact, I miss a tutorial explaining how I will keep my sites updated.

In user manual chapter 8, it is explained that when there is an upgrade this mean that the DB layout can be affected.

In this case, you have to re-install the extension in each slave site to let the extension proceed with the DB layout modification in the slave site.
The JMS tool menu does not know the specific stuff that is present in the extension to perform the DB modification. This is the reason why you have to re-install the extension in the slave site to let the extension detect the modification and proceed with the upgrade.