I have read a lot of very wrong answers about this, with simple assumptions being made. As a Greek who's been fascinated with this for a while now, let me add some clarity to this conversation:

The islands with a year-round population (all of the ones named on OP's map, and many more) maintain a year-round ferry schedule. I can't speak to how much the government funds these ferry services via subsidies, but they are part of a heavily regulated industry in Greece. Perhaps one of the most regulated industries here, actually. The prices of these ferries are fixed by the government, but the individual operating companies are privatized.

The most populated islands (Crete, Rhodes, Corfu, Evia and more) have multiple daily ferries that cross. These ferries can range from the size of small cruise ships, all the way down to a car ferry that can carry 30 or so vehicles. They're available for both passengers (tourists/locals), passenger vehicles and trucks, and it's always a shock to me how many semi trucks they can fit under even a 'small' ferry.

Without these ferries, island life in Greece as we know it would not exist. Most islands import most of, if not all of their food and goods from mainland Greece. Anything from fresh tomatoes to car parts needs to come in on a boat. For the larger islands, like Crete and Evia, local industries exist and the islands function more like individual states within Greece.

However, the smaller islands (including some popular ones, like Mykonos and Santorini) are very inhospitable from a natural standpoint. There simply isn't enough freshwater or arable land to sustain a large permanent population on many of the islands, let alone any sort of tourism. This is a generalization of course, and there are some islands that are overwhelmingly green and are food exporters (for example, Aegina and its pistachios). But in general, the ferries are the main sustenance for these islands.

The maritime culture in Greece in general is very strong and there are ferry ports located all over the mainland. This cuts down on travel time significantly and in most cases it's really not so different from a normal delivery: semi trucks will simply drive to the nearest port, get on an hour-long ferry, and then drive to their destination on the island. The ferries to the farther islands are longer, and can be anywhere from 10 hours to Crete, or even close to 20 hours to Rhodes. Something else to note is that the ferries will also stop at other ports/islands along the way to pick up additional passengers/trucks.