You need to post the modelnummer you have G7 is an entire product line the specs can be different in each model .
Most likely you can install 1 M2 ssd to tell you more i need the productnumber to be able to look at the manual.
I checked based on the receipt my brother sent be way back its written as Dell G7 7588..... specs is i5 8300h 8gb ram GTX 1050 4gb 1tb HDD . Thanks
Yes that's it 7588 you can install 1 one M2 ssd(2280) sata or NVMe .The sata limit is 128 GB and the NVMe limit is 1TB however those limits indicate that dell did not test larger SSD's there is no real reason an ssd that exceeds the limit will not work.This model also has 1 2.5 inch drive this is a sata drive (max 2TB)
You can find this information under the storage heading in this manual