According to statista.com, US Q4 GDP growth rate was 2.6% over the economy in Q3. However, why is the annualized growth rate for the US therefore not be a sum of all 4 quarters if their definition of QoQ growth is basically how much the economy grew over the previous quarter? i.e. why is according to statista, the GDP growth of the US in 2018-19 only 2.9%, and not 2.6 + 3.4 + 4.2 + 2.2?
Because those data are Seasonally adjusted annual rate: https://en.wikipedia.org/wiki/Seasonally_adjusted_annual_rate
Annualized growth rate does not mean the same as annual growth rate.
An annual growth rate looks at YoY. A quarterly growth rate looks at QoQ.
The annualized growth rate takes the quarterly growth rate and raises it to the fourth power. Essentially, the annualized figure is saying "if the economy maintains the same current quarterly growth for the coming three quarters, we will have an annual growth rate of this number."