From conversation on parent task in T90687#7246210 on and internal Design team component work and discussions before the base spacing values are currently undocumented.
Proposed is a 4px base as minimal spacing unit. And multipliers depending on context.
WikimediaUI Base is following the 4px multiplier already, OOUI has gotten closer over the years in WikimediaUI theme, but still features some exceptions.
Wikit features 0.125em as well.