Could you briefly describe how to sample frames from a video?

I run the code and find out that, say I have a video with 30fps and 300 frames in total. In `__getitem__`, 64 frames would be sampled first from 300, then 8 would be uniformly sampled from 64.
I was messed up with the code. Could you briefly describe the strategy of how to sample frames from a video? That would help a lot! If possible, could you also also describe the sampling behavior during training and during testing?
Thank you!