In dramaturgical terms I’d define what you say here as
For example, lawyers starts a new office and falls in love with secretary is just a premise, a starting point.
it’s a plot summary or precis.
And then to take a few of your points:
But it turns out that said lawyer has a strong imposter syndrome that makes him doubts everything he does despite his externally cocky attitude,
these are character attributes and are not necessarily writer-controlled and could vary wildly between the writers’ intent, the directorial notes and the actor portrayal.
while the secretary is actually a down on her luck law student who got emotionally abused by her previous boyfriend who is now the new hire on a high tier firm that is the main competitor of the protagonist’s firm.
debatable about whether this is plot. If we see (or hear about) this happening relative to a turn of a beat or a block of the objective to a character in a beat - then, that’s plot otherwise its exposition
Main guy shows interest on the secretary but they have a fallout due to her wariness and his insecurity.
If we see the main guy making a move, this is plot, the reason for her rejection is an attribute or expository, but is not specifically dramaturgical
Then the opposing firm starts a hostile takeover by stealing clients and lawyers.
Technically this wouldn’t actually be plot without us seeing the opposing firm (presumably by synecdoche of seeing a character from the firm). So if we meet people from the competition firm it’s plot, but if we don’t then the plot would be more accurately described as “characters X, Y and Z leave the firm” and less accurately that “the competitor steals the characters”
the reason for being this specific is a) artistic - that drama (including comedic drama) relies on character relationships and dialogue and b) the process of turning writing to performance to product is a large and refined one that requires adherence to these principles to function