Where can I hide powered by Nopcommerce and nopcommerce version in admin section
\Views\Common\Footer.cshtml file for 3.50/3.60 versions. You need manually to remove it.
3.70/3.80 you can remove it by going to admin area > configuration > settings > all settings > set 'storeinformationsettings.hidepoweredbynopcommerce' to 'true'.
For admin area, not sure.
Btw, you need to purchase removal license key.