Entries of the likelihood function

Im just getting started with PYMC3 and have a rather basic question regarding the likelihoodfunction.

If one defines a simple normal-distributed prior(expected distribution), this distribution is used as an input to the likelihood-function. The distribution for the likelihood however, is this just a belief of what distribution the observed data is?

also, is the sigma used as input to the likelihood-function calculated “by-hand”(standard-deviation calculation)?

Thanks for answering