Agree with vmroyale on number 2.
For you question 1, I am assuming you want to know the version of ESX version running on the host. You can find out by selecting the host after logging in to vCenter. You should be able to see the build number as well.
Or if you want to know the OS for VM's running on the host, Select host ->Virtual machines tab and enable Guest OS option by right clicking on the header.