Do I need to carry snow chains when driving in Spain in December . Is it law ?

Here's the advice obviously depends on where you are in Spain.
https://n332.es/prepare-for-winter-driving/

You don't have to carry them in the boot everywhere, but if you drive on a section of road where you must have them fitted (e.g. you pass a blue circle with picture of a white tyre fitted with snow chains) and the chains aren't fitted and your car doesn't have winter or all-season tyres either you can be fined 200 euros.

Fit some Michelin Cross Climates and you will not have to give this a seconds thought again (google cross climate reviews)

I'd check reviews for all-weather tyres and go for the ones that show best performance in summer conditions (sadly at the expense of winter weather specs) and just go with those. They will be good enough for winter but won't deviate much from regular ("warm weather") tyres.

no.

unless you are in a high area. where it snows. or gets icy.

they are 35€. get a set amazon primed and bring them just in case ?