It's one of the oldest wars of all time: the everlasting battle between the new and the old. Things were very much in flux in the late 1800s, creating a race around the track to see who would burst through the finish line first.
The thing is, it's not just as simple as the van Rhijns against the Russells; there's much more that comprises the two houses, including those in the families themselves as well as the staff that operates beneath them.
The way that each house spends its own money goes directly into how they wish to come off as people. For one, the Russells tout their money by dumping much of it into the creation of their mansion, in hiring a French chef to be on duty at all times, and by their actions outside of the house itself. This includes how George Russell (Morgan Spector) willingly bought up the entire merchandise at a charity event simply to make a social statement.
On the other hand, the van Rhijns live in a beautiful, stately home, though it looks like nothing compared to the Russell's bustling estate right across the street. They have a devoted staff who seem genuinely more invested in the family itself than the Russell's staff does, and while they don't have a French chef cooking all their food, their meals are most likely just as well-made.
Where the two houses converge, however, is the way in which they raise their children, though, on the van Rhijn's side, this also applies to Agnes (Christine Baranski) and Ada's (Cynthia Nixon) niece, Marian (Louisa Jacobson). Marian found someone that she loves but that Agnes disapproves of; Mr. Tom Raikes (Thomas Cocquerel) isn't quite up to Agnes' standards, and she believes he will want someone with more money than Marian. Because of this, she practically forbids Marian from seeing him.
In the same way, Bertha Russell (Carrie Coon) is extremely protective over her daughter, Gladys (Taissa Farmiga), making her wait too long to officially come out in her debutante ball. Her father sends other suitors away, paying them off by providing them with lucrative jobs in his industry.
Speaking of jobs, Mr. Russell also doesn't approve of his son's sudden interest in a new career path: architecture. Wanting Larry (Harry Richardson) to continue working for him and eventually take over the company someday, it's going to be tough for him to convince his father otherwise.
Basically, there's no pleasing either of the household heads.
The main thing that separates the two houses is that Agnes doesn't care about Bertha's attempts to one-up her and those around her. Because Agnes is part of the old money crew, she doesn't need to one-up anyone—she's already above Bertha. However, as she watches Bertha climb further and further up the ladder, the air up there is starting to feel a bit suffocating to Agnes.