How do they do this when other makes Of this age are written off because the failures make the car economically unviable ?

Maybe because it was made in France?

And how many miles has this Yaris covered?

Try this - en.wikipedia.org/wiki/The_Toyota_Way - or the book of the same name. Much of it is applied common sense. I have worked with tier 3 and 4 suppliers who value the opportunity to work with Toyota. Suppliers to Mr T are challenged and targeted to reduce waste/cost but are supported, encouraged and treated fairly (not the same as equally) with a cut of the savings coming their way and assured business if you play the game. This includes open-book costings, a big step for a supplier.

Those same suppliers are less than positive about certain other manufacturers who drive them on price, get the lowest possible price and then approach other suppliers to see if they can beat it. Having agreed a deal for a certain number of units, in the case I am thinking of it was 500k units and £Ms in value, the buyer then bought only 350k and refused to accept the contracted quantity. The supplier was depending on the volume to achieve the cost and lost money of course.

An even more egregious case was a major manufacturer who worked with the supplier through the pre-launch phase to develop the particular category of products they supply. It is usual for suppliers to have their staff work with the buyers team to develop product designs and specs. having worked for two years on a NMR (new model release) the buyer then took the specs and hawked then around other potential suppliers. If you knew who the major car manufacturer was you would probably not be surprised.