Do all electric ovens cooktops ranges and clothes dryers operate on 208v as well as 240v?

No, use only the voltage for the appliance that your utility system is supplying to your residence. If your system is 208 volts use 208 volt appliances. Likewise if the utility system is 240 volts use only 240 volt appliances. 208 appliances on 240 will overheat the appliances and 240 appliances on 208 will not produce the wattages that the manufacturer recommends for cooking and drying.