'I was wondering how the "combine all the health points of bases grouped together" thing works in practice?'
You create units from 2-4 bases (or companies as they are known in the rules). Each company has a 'Resolve' value, for example a Formed Company has a resolve of 4, thus a unit of 3x Formed Companies has a total resolve of 12. We use counters to keep score of loss of resolve.
At the price, it is very hard to find a better system than Fantastic battles and I have tried lots.