It's a figurative use of the word strand: just like multiple (fiber) strands make up a thread, multiple strands (factors/subplots) may make up a whole (concept/plot).
"There are several Khasi tournaments played through the year, in all
villages, all unified by two common factors [all sharing two common strands] - the prize goat and that all
players are tribal people."
ODO:
strand[2]
NOUN
1.3 An element that forms part of a complex whole.
‘He never adequately connects the several different strands he's
weaving into a cohesive whole theory.’