Gst on 100% software export firm

Hello,

The firm is 100% Software export Unit which is Zero rated with Turnover of 50+ Lakh.

# Do We need to registerd under GST

IF Registered

# Do we need to file Nil return and claim input ?

# Do we need to pay tax and then clain it with other and claim input ?

# For any service from third party if he is not registerd under GST (due to less than 20 Lacs). Do we need to pay its GST ??

# What is Reverse mechanism, how it works ?

Thanks in Advance,