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This is a Euphorbia, and is going through a process known as "corking". It is normal, to an extent. In this case, this would be considered excessive corking, and will signify significant stress.

I would give them shade from the afternoon sun, and make sure that the soil is NOT clay and well draining cactus mix with plenty of sand.

Overwatering and too much sun/heat will cause excessive corking, so I would ensure that neither of those is happening.

This article will explain more about the natural defense systems of cacti: https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/saving-sunburned-cactus-plants.htm

It appears to be a Euphorbia of some type. There are many varieties, from plant to cactus forms, to trees with deadly sap, making this one of the most broad genus on the planet.

You can cut it down, and pour boiling water on it once or twice per day for a few days. This will completely kill it off without making the ground toxic.

General care is similar for most Euphorbias, although there are many species of various sizes and forms, so there will be some variations with planting, potting support etc. Here is more information:

https://www.gardeningknowhow.com/ornamental/cacti-succulents/euphorbia/growing-euphorbia-plants.htm

That is one of countless Euphorbia. The Genus ranges from trees to small succulents, all of which have a toxic sap. Some are more toxic than others.

https://www.gardeningknowhow.com/ornamental/cacti-succulents/euphorbia/growing-euphorbia-plants.htm
My best guess will be Euphorbia helioscopia.

This happens, most often, with overwatering. Since there are stones on top of the soil, it will be very difficult to know when to water. It is very important to let the soil dry out, very thoroughly, to at least halfway down the container to prevent this.

There is no cure, nor will the lesions go away, but you can prevent further damage and new growth from sharing the same fate by changing the watering habits.

This article will offer more information:

https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/cactus-scab-treatment.htm

You might try contacting this California native plant society and see if they recognize the plants:

https://www.sacvalleycnps.org/26-drought/71-native-plants