In conv2

6 -> refers to the number of in_channels, after first conv your output had 6 out_channels, so they become input in conv2
16 -> refers to the number of out_channels. It can be any number. It is your choice. You can set it to 100, 10 anything.
3 -> kernel_size again is your choice. But kernel_size value 2 and 3 are common, so don’t need to experiment much