Can you explain the idea of indifference in the song "Sick Boy" by The Chainsmokers?

The theme or idea of indifference in this song is introduced to listeners at about the song's halfway mark. The following lyrics are likely what this question is specifically asking about.

Welcome to the narcissismWhere we're united under our indifference

The song itself is a fairly scathing indictment against a societal culture that has huge narcissistic tendencies. A narcissistic person is someone who is extremely self-centered and has an exaggerated sense of self-importance. Essentially, a narcissist is infatuated with his/her own self. This kind of self-important belief can be seen in something as common as the "selfie" picture. The general population's focus on the individual self and taking pictures of yourself is what led cell phone manufacturers to start producing phones with the front-facing camera. People could then see what the picture of themselves was going to look like. Then additional selfie toys began to emerge. Selfie sticks allowed users to get better/different angles of themselves and some wider angle shots, but the goal was still getting great pictures of yourself. The posts on whatever is the chosen social media platform echo this idea of being self-centered. It's common for posts to be self-centered posts. They highlight what great thing a person did or is doing. Many of those posts then receive feedback from other viewers in the form of a "like" button. The more "likes" that the post gets, the more people that saw the post and enjoyed it. Getting the "likes" is a sort of rewards system for individual users, but a problem is that some users tie the number of likes to their own self-worth.

How many likes is my life worth?

This is a depressing way to live because it forces people to feel like they always have to present an unrealistic, always awesome version of themselves online. It can become a consuming way to live life, and the song likens it to a prison or religion.

Make no mistake, I live in a prisonThat I built myself, it is my religion

What's interesting about all of this self-love is how it affects how people view other people. Users might be clicking "like" on someone else's post, but their main concern is still likely their own personal number of likes. An attitude is created that basically says, "Your stuff is cool, but mine is cooler." And if that can't be said, a common response that I see and hear is "whatever." As an interjection, "whatever" is defined in the following way:

Said as a response indicating a reluctance to discuss something, implying indifference, skepticism, or exasperation.

The song "Sick Boy" says that we are "united under our indifference." That sounds great for a second. Being united is a good thing; however, united through indifference isn't a good thing. A group that shares only their lack of care for anything isn't a strongly bonded group. This is because individual members of the group care more for themselves than they do the group. The song goes on to highlight how this indifference actually creates division. That's why people can pick whatever side is most convenient for them at the time.

And we can pick sides, but this is us, this is us, this is