It is a function of the total quantity of the currency at any given time, that currencies distribution across the economy, and the supply of the products/services it is being used for trade.

Then the banking system gets involved and it gets a bit complicated, but essentially any currency that is used for fractional reserve banking will experience a slow (or fast, depends on regulatory control) expansion in its total quantity over time.

Of course nobody would perform unregulated fractional reserve banking with cryptocurrencies now would they? Well actually, this is exactly what tether, luna etc are/were doing under the hood.