Chika actually had multiple personality disorder and everyone in Aqours was just an imaginary friend of hers.

Riko was a personality that shown up last because Chika wanted a change in her life. Chika was obsessed with Muse, so she subconsciously created a dream girl that was a good singer, great piano player and came straight from Otonokizaka. Season 1 was all about Riko becoming close with her creator.

Now think back to certain scenes. Why would everyone near the end of Episode 1 of S2 gather to the same place that Chika went to without her even informing them? Then think back to how Chika automatically knew what the 1st years were doing during the mini Saint Snow arc. The final episode was all of those personalities becoming one within Chika and she finally moves on with her life. The "radiance" that she was seeking was becoming complete with herself.

And don't forget. The LLSS director said that he left certain things open ended in the series for the fans to interpret however they wanted.