Understanding network addressing

I am currently studying for my ccna and I’m relatively new to the whole thing. So sorry in advance if it is obvious or this is a dumb question. I’m confused on the way networking addressing works. For example let’s say we have a typical home private network 192.168.1.0 and the subnet is 255.255.255.0, is the the first three octets customizable? I have seen some of my friends networks are 192.168.0.1 or 10.1.1.1 and mine is 192.168.1.0. Could I go home and customize my internal private network to be 192.168.15.1? Or could I make it a class A network and put 10.0.0.1? Or even modify the subnet to whatever I wanted? I understand it is not feasible for a class A since it would be too many addresses but just curious. This has been itching in my mind and I cannot modify my own network to see since they’re is other people using it constantly. Again sorry if it is a dumb question. I’d rather ask a dumb question then not know and be wrong. Thanks in advance