First I want to start with a disclaimer which is that all of this text is true under fiat currency rules, which holds since 1971 when Nixon cancelled the gold standard.
A central bank can create its own sovereign currency, and also destroy it.
The way it increases its currency quantity is by giving it to another entity, which can be as a gift, a loan or as a payment.
It can cause deflation or deflation at will, but it comes with a lag, since it usually takes time for the newly created currency to spread through different asset classes.
When a central bank increases the quantity of its currency, it causes a delayed devaluation of it, unless it is coordinated with foreign central banks which create suitable amounts of their own currencies, in order to maintain ForEx (Foreign Exchange) rates stable.
Central banks ' authorities are usually subject to legalization of their countries or superior entities (the IMF and WTO are not completely independent either) .
Central banks are not necessarily entirely public or even partially public at all, and an example of the former is the USA's Federal Reserve, which 6% of its profits go to its share holders which are private entities.
Until the initiation of the Federal Reserve in 1913, each bank in the USA could issue its own currency, which was debt based and backed by gold, which is not a requirement for a currency, but was the case back then.
