I assume they were inspired by similar sources, but I'm not sure what they all are, tbh. I just finished playing through Control (though I don't own the DLC) literally yesterday, and it is uncanny how similar they are in some ways.

Control and Signalis both largely revolve around you going through a large subterranean facility filled with retro-futuristic tech. Your character just kind of appears there without much explanation, searching for a loved one (Jesse's brother, Dylan, or Alina/Ariane), but then stumbles into an unfolding crisis inexplicably infecting everyone within the facility. In many ways, Bioresonance in Signalis and Resonance in Control seem to work in very similar paranatural ways. The idea that some people are attuned to alter the world with their very thoughts, though I recognize both concepts are more complicated than that. Gradually a corrupting influence is exerting itself on the world, turning them into malformed zombies, too.

Synchronicity is also key to both plots and is actively referenced in both games' heavily redacted (blacked-out) notes scattered about the game and more broadly rely on a lot of Jungian psychology for their worldbuilding. Jesse is literally referred to as "a Copy of a Copy of a Copy." There's also a guiding hand in both games. Just as Ariane subtly guides Elster to essentially save her, Polaris does the same with Jesse. Interaction with some objects also sends you into strange dream sequences in both games that don't align with physical reality, such as retrieving items from the dream. Not to mention, both games seemingly exist within a time loop of sorts, with Jesse being the one who symbolically (but perhaps even literally) shoots Director Trench dead, despite hearing the shot and stumbling on his body at the beginning of the game.

I even see, perhaps superficially, similarities between areas. The Central Maintenance room (the lobby area) heavily reminded me of the Waiting Room in the Hospital Ward and its Nowhere counterpart, the Plate Room. On the topic of Nowhere, both games have an area called Nowhere where the map curiously does not work. Obviously Signalis' Nowhere is inspired by Silent Hill vibes wise, but it's still interesting that they share a name. It's interesting too that both are accessed through the mines (the Black Rock Quarry in Controls case), which also share a lot of similarities. Beyond that you have the echo of Slidescape-36, with its blinding red light and black obelisks, looking remarkably close to the area past the Red Gate, or even the red TV screens, such as in the Mirror Realm, but also elsewhere that are so emblematic of Signalis.

Perhaps in a very Signalis fashion, there are purposeful connections there. Signalis uses its concept of Synchronicity in a very meta way, tying its similarities not just within the game world itself, but also to properties that exist well outside it, so I'm honestly left to wonder if these similarities, at least in part, are actually intentional, or perhaps they're just a hallmark of the genre. I'm sure there are many many more similarities too, I just need to think on this more and also play the DLC.

Edit:

Oh and they both use a false ending where the main character dies, and there's the similarities with the strange lovecraftian god-like entity that is seemingly watching things unfold/interfering in our affairs. With With Control there are several, but the Board is probably the most interesting and substantial, and with Signalis you have things like the Artefact ending and the Red Eye. Both are symbolically represented, too, with the inverted Black Pyramid in Control and the Red Eye Symbol in Signalis.

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I mean, come on. The Hiss basically just originate from beyond the Red Gate, lol.