I am interested in developing or playing around with the source code of bitcoin. I had question regarding this. I am interested in only having a serious discussion about it, so made the thread self-moderated.

1. What source code do most of the big altcoins use to develop their coins ?  On researching this, it seems its litecoin, but is that what all the developers of coins even like ETH start off with, or do they also start off with bitcoins  ?

thank you!