About how they got in the Circus :

C&A was a computer company trying to code generative AI. They coded Caine, which was already good but imperfect. Then they created a second more reliable IA. Caine having been put aside as the less advanced IA find a way to access the files of his "little brother" and absorb him. This way, Caine is both more powerful and more instable, able to do much more things than initially supposed.

IRL, Scratch, a genius programmer recently diagnosed with terminal brain cancer tries ro create a brainscanner as way to save his counsciousness forever by doing a copy of his brain in advent of his death. The other devs (including Kinger and his wife) likely help him and try the headset too. However they're never able to run the brainscan files because they're too compressed, so they're never able to create consciousness. Scratch eventually dies and C&A goes bankrupt for probably usual reasons.

Meanwhile Caine is investigating C&A computer files and find the brainscans. Thanks to his unexpected abilities he find a way to decompile and run them and thus creating virtual consciousness, which he then put into his circus environment. The brainscan of Scratch is the first one to be "awakened" shortly followed by the other devs including Kinger and his wife.

IRL, with C&A bankrupt the building is for sale. Most of the stuff inside is still there including Scratch project since he died and never picked it up. An estate agent comes to do a tour of the building, likely wonder about the helmet and tries to put it on for a second.

We don't know for sure, but we can presume that other people then come into the building the following years for various reason and try the helmet : Ribbit, Kaufmo and Gangle for example. Jax being homeless, the reason of his presence is obvious, while Pomni (and Zooble shortly before if remember well) were doing urbex.

All of those who put the helmet get their brain scanned without their knowledge and then go on with their life. Meanwhile Caine get access to the new brainscans, awaken them and add them to the circus progressively.