But if the match goes to the 3rd, 7th, 18th ..... extra hole, where are the strokes received?

This is why the Stroke Index system was designed for matchplay not strokeplay. Both CONGU and the USGA have recommended procedures (essentially the same) for allocating the SI.

For example they say that SI 1 and 2 should not be allocated to hole 1 or 18 ( or 10 or 9 if some matches may start at the 10th). The odds should be allocated to one half and the evens to the other. Amongst other things it prevents a player giving 2 strokes finding himself giving a stroke on the 1st extra. However, it also lets the player receiving 6 get, on average, 1 stroke in every 3 played. There is too much to include here but suffice to say it is not primarily about relative difficulty.
BUT many clubs do not follow the guidelines.

You will appreciate that SI is irrelevant in strokeplay as the strokes are deducted from the gross score at the end of the round. Except stableford where swings and roundabouts come into play - you may get a stroke on an 'easy' hole but don't one on a harder one.