When a central bank creates new paper currency, it is sold off to the commercial (high-street) banks, a process known as seigniorage.

The commercial banks then distribute this money via their cash management division through all the channels they have, predominantly, ATMs and Over-the-Counter.

If the new currency in replacing old currency, then customers usually queue up at banks and exchange their old currency for the new one (happened in India very recently).

In most cases, the distribution is digital. So central bank literally just creates money on the corresponding bank ledger and denotes an increase in money supply.