Some newbie questions

Hello,

Could you plz answer the following questions of a newbie:

What is a good way to start understanding backend(postgres) code? Is
there any documentation available especially for developers?

What is commit log and why it is needed?

Why does a replication solution need log shipping and why cant we just
ship the transaction statements to a standby node?

to be continued  ... ;)

Thanks,
Srinivas