The "normal" used to compute anomalies: the average over a 30-year reference period, usually 1991-2020, for each place and each day or month.
Indices
A climatology is the long-term average of a variable for each calendar day or month. The World Meteorological Organization's current standard period is 1991-2020, and many products in this app use it:
The Koh Samui rain and temperature normals (ERA5, 1991-2020), used for "% of normal".
Climate Reanalyzer's daily SST charts.
Some indices use other references: NOAA's ONI uses centred 30-year base periods updated every 5 years; MEI.v2 uses 1980-2018; the BoM SOI uses 1933-1992.
Why it matters: the ocean has warmed since 1991-2020, so a fixed old baseline makes everything look "above normal". That is one reason NOAA now uses RONI.