@Ramón Marín there's not built in synching in Trello. To an extent you can use Butler for this, but if you're using it with any degree of seriousness you'll exhaust your quota unless you're on enterprise (which requires a minimum of 20 seats).
You can use Unito which is a pretty good generic synching tool between boards. You also need to think about what it is you want to synch.
For example, do you want to have a list on your main board for each project that shows you the cards in "todo" for each one? My preferred approach is to have a "board of boards" where each sub-project board is linked from a card in the overview board.
In that scenario it can be good to have, for example, a checklist on the overview card that automatically generates each day/every few hours with each item linked to a card in ToDo, with the items being checked off as they are moved to the Done list in the sub-project board.
This is the "more interesting" example I give in my article introducing Trellinator, the open source Google Apps Script automation framework I created for Trello:
Hello Community! My name is Brittany Joiner and I am a Trello enthusiast and Atlassian Community Leader. I'll be sharing with you a series of videos with my top tips on how to best use Trello...
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