As long as the FIA doesn’t disclose the details of the deal with Scuderia, one can only speculate: What exactly did Ferrari do differently from the competition in 2019?
However: The technical directives that apply for 2020 provide a hint. It clearly regulates what the teams are not allowed to do. Already in 2019, it was made clear in TD35 / 19 that tampering with the fuel flow sensor is illegal. Well, duh!
The focus for 2020 however: The directive TD / 042-19 stipulates a second sensor to measure the fuel flow. It is placed behind the first, it is encrypted and collects data on a different frequency that only the FIA knows.
This is to prevent the teams from manipulating the signal and injecting more petrol. In order to really rule out any fraud, the sensors will only be placed just before the race weekend and only for those teams that have been randomly selected. This means that manipulation of the hardware can be completely ruled out. Please note that only 100 kg of petrol per hour are allowed.
In addition: According to paragraph 7.9 of the technical regulations, only an additional oil tank is allowed, which can’t hold more than 2.5 liters. The new rule makes it virtually impossible for oil to be injected illegally into the combustion chambers in order to increase performance.