Mirrors are clones of pre-cals. As example, in episode 4 we can see Hope's mirror, 390. Every pre-cal had his/her own mirrors. The mirrors were under the "care" of Abal Ammash and Noam Peretz. Mirrors, like Pre-Cals, are intended by Treglozov to be used as human hard drives via dna digital storage. Every mirror lives in tiny little cubbies in a room. When Abal and Noam are given Directive 95 (The deliberate murder of all mirrors and recalibration of all Pre-Cals), they can’t carry out the directive and order Mammoth to carry out the Directive. Before the directive was initiated, Zager freed the Mirrors. This is why every time Mammoth kills the mirror the player is guiding, a new “Hope” mirror shows up that the player guides. There are multiple ”Hopes” running around trying to avoid Mammoth.
At the very end of the Episode 3 credits, right before Mirreile and the Prizrak had a radio conversation, Hope says "I give up" and it sounds like she walks away. 390 probably was just watching her from afar, and once Hope left 390 went to pick up the phone and key.
Metamorphosis is using the children as super-dense data storage, calling them “mirrors”. Pre-Cals are special mirrors that live in the Facility that, for one reason or another that could cause them to become “disloyal“ to the Overseer, require “recalibration”—or “reformatting” in hard drive terms. Part of this includes memory loss, which is what leads to this innocent, child-like personality we see in the 390s from episode four. The Hope we know is a specific 390 clone that requires pre-calibration (so, a “Pre-Cal”) because she read one of the poisoned Manifestos left by Zager. Episode four ends with the last clone getting caught by Treglazov in her escape. He shoots her dead, and picks up the phone you’ve been using to communicate with Hope and the clones.
Episode five brings us back to the Hope we know—scars, collar and all. After finding the elevator in episode three, she was captured by Treglazov (and presumably lost her phone in the garden). Treglazov now has the phone he found on the clone.she is in his office with Derringer there to keep her from running away as the Overseer explains to Hope that she is 1 in a series of clones.
Close to the end of Episode 5 when Hope Overloads Terminus, There are twelve long audio clips in total here—some are about Treglazov and how he built Terminus, and others follow other past 390 clones as they try to live in the world. One, named 390-A (“Anne”) is in an asylum in Norway. Another, named 390-G, is a mother of a young boy in Washington. All of these women have Hope’s voice, and discuss flashbacks to their days at Metamorphosis. Note that these audio clips are not from the future—if you look at the date on some of them, they take place before Hope escapes (in 2020), so they must be “failed” clones or something of the sort that were sent back into society...for some reason.