allow me to sum up interpersonal relationships between all 8 billion people in one sentence:
you can’t

but, in a nutshell… physical attraction is usually important, so is romance… usually a commitment to this person in terms of time and future plans where they become more like family (and maybe eventually actually family if you have kids)

there’s such a crazy spectrum of the ways in which people could get along and consider themselves “in a relationship”.
every rule or tendency is constantly broken, it escapes definition.
in general, it begins with attraction and flirting, or it’s cemented as a friendship…
and, poetically, i’d say it’s a relationship when they start to see themselves as an entity, and of course others will see them like that too… a sort of hive-mind develops…
sorta like when celebrity couples get a combo nickname…