are all plastids endosymbiosis
Plastids show Endosymbiosis, or are endosymbionts
Endo = Inside
Symbiont = interaction together
A free living bacteria entered a cell, where both of them then benefitted each
other.
With a little modification in the bacteria, it permanently stayed inside the
cell, and became known as "Plastid".
Another endosymbiont is the Mitochondria, which came from a free living
aerobic bacteria.
Source: https://en.wikipedia.org/wiki/Plastid_evolution
", in order to live in symbiosis, the eukaryotic cell that engulfed the
cyanobacterium must now provide proteins and metabolites to maintain the
functions of the bacteria in exchange for energy."
-https://en.wikipedia.org/wiki/Plastid_evolution#Primary_endosymbiosis