usually figures can be washed with plain water
you can even use diluted dishwashing liquid if you wish to, i use it to remove stains especially fingerprints along with a sponge, it doesnt damage the paint. Nothing too concentrated, diluted liquid. Scale figures are protected by a layer of coating, so soap and stuff are unable to damage it easily. Chemicals that use water as solvent are unlikely to damage a figure, but beware of chemicals using ether, alcohol, turpentine or thinner as solvents. And remember that this protective coating is not present on cheap prize figures.

For freezer, it's to prevent or slow down a type of chemical found in PVC from seeping out of the figure and evaporating, called plasticizer. Sometimes when u buy a new figure, the clear plastic wrapping the figure is a bit oily, that is plasticizer. Plasticizer keeps a figure's parts flexible enough, so that the PVC material wont be brittle. But under constant exposure to warm temperature or sun it seeps out of the figure making the figure oily, then evaporates, leaving a sticky residue behind. As plasticizer leaves the figure, the pvc structure of the figure becomes more brittle and easier to break, that is why you hear cases of broken nendoroid joints (especially at the neck where weight stress is highest), the joints had weakened due to this heat effect. For older figures which do not have ABS and other composite materials, the heat effect softens the PVC, combined with uneven weight distribution of the figure, making it bend or lean in the long run.

Newer figures are made from a composite of PVC and ABS, and/or ATBC-PVC, which is stronger and more heat resistant. But the plasticizer evaporation can still happen - and to prevent or minimize that we shove the figures into a freezer once in a while. Doing so both prevents evaporation of plasticizer, and strengthens the PVC structure which has been softened or weakened by heat. Each figure only needs to spend 10-15 minutes in a freezer, not too long. You will notice a pvc figure become a bit harder after refrigerating it, that's normal and probably a good thing.

When you compare figures of similar sizes, some feel quite heavy, some feel quite light. The heavy one has more PVC and less to none ABS. Those extremely light ones have a higher percentage of ABS, and also more expensive to manufacture. If you have touched or built gunpla before, the tough rigid plastic you feel in your hands is ABS plastic. That material is way more heat resistant and does not bend under warm temperature. Nowadays usually you will notice Good Smile and Alter's scale figures are relatively light, and Kotobukiya's to be really heavy. The light ones are actually a sign that the figure is of good quality in terms of material composition.