Here is what happened as far as I know.
Leclerc was on pole, Hamilton 2nd and Vettel 3rd directly behnd Leclerc. This track runs clockwise with long run to a righthand turn one with the pole and 3rd grid spots on the left hand side of the track. Ferrari hatch a plan for Vettel to get past Hamilton. Leclerc would give Vettel a tow on the back of Leclerc's car. This would require coordination between the two Ferrari's as Leclerc would have stay on the outsid line of the track down to turn one as long as possible before breaking for the corner and allowing Vettel to slip into 2nd place behind him.
The trouble with the plan was it worked a little to well because Vettel got an absolute beast of a start and slipped pasted Hamilton before getting into Leclerc's slipstream. Once he had the tow from Leclerc's car, he used it dive past his teammate instead. Ferrari wanted to swap the drivers back round because Leclerc only lost the lead to Vettel because Leclerc kept up his end of the bargain.
This all went tits up for Ferrari when Vettel's car broke down bringing out a virtual safety car and both Mercedes took this opportunity to pit and come out ahead of Leclerc who had already pitted earlier in the race.