Does it have a battery inside?
No, it does not have battery inside .You need to connect it with a power source(wall charger, power bank) via Type-C cable for it to operate.
Can I use it with Tablets as well?
No, it's designed only for smartphone and would not fit the tablets.
Can I use it with mobile back cover?
For best efficiency it's recommended to remove the back cover of smartphone.Back covers prevents heat dissipation process which defeats the entire purpose of cooling down phone's temperature.