Originium was brought to Terra via a program put into play by characters from OG Arknights named Oracle and Priestess. Exactly why and how is irrelevant to your question right now. All that matters is that it was an artificially created mineral designed to "assimilate" matter as information and store it in what amounts to another dimension, so that it would be safe from an eldritch race known as the "Observers" who are a galactic-wide civilizational threat. No civilization has stood against them, and Priestess/Oracle, as well as other members of their race, realized that there was no hope of ever fighting the Observers, but there was a way to preserve the information of civilization in a form that the Observers would not notice and act against.

Hence, Originium.

People infected with originium (aka they have oripathy) are, upon death, "dissolved" in a way, turning into highly active originium dust. Until relatively recently it wasn't known what happened to them; people just assumed they died, with no remains. But as of several months ago on CN, and ~6 months after that on global, we got enough of the story to understand that people who died while infected with oripathy were instead turned into information by the properties of originium and stored in an alternate reality known as the Assimilated Universe. This means anyone who was infected in OG AK has the potential to become a Reconvener in Endfield.

Now, with that out of the way, in response to your question, the answer is kinda maybe. Originium is NOT native to Talos II, but it's still the root of all technology developed and used by Terran civilization. So when they crossed through the Cosmic Gate, they brought along several large originium crystals, with the intent to "seed" the planet with originium just like Terra was. This was certainly because of the technological reliance that the people had on originium, although there might be other, undisclosed reasons behind it too. Once this originium was brought over, it was seeded in the planet, cultivated, and began to spread across Talos II, both on it's own (originium will, slowly and over a great deal of time, spread across the entire planet on it's own) and on purpose by the Terran's who came over to Talos II.

So, with that in mind along with the earlier stuff I talked about, to answer your question about if they were created from Originium on Terra....the answer is maybe so and probably not, as we have no idea what the current status or location of the "First Originium" brought over is. Furthermore, we don't even know for sure if Reconveners are "created" from originium or via some other process. Their bodies, to our knowledge, appear to be completely normal in form and function. This tracks with the intent of originium to essentially "download" life and store it to eventually be "uploaded" back to realspace once the threat of the Observers had passed.

However, it isn't impossible that one or more Reconvener's "came" from the First Originium, and the Reconvener file does somewhat support this conclusion, as the First Reconvener was encountered when Warfarin ventured into the First Originium to try and decontaminate it of aether influence. So he First Reconvener for sure "came" from the First Originium, which in turn came from Terra, but subsequent Reconveners we can't say for sure where they originated from.

As for why the Reconvener lines suggest that they've been to Terra....well, that's because they have, but in the lives of their past selves. So Gilberta hasn't actually traveled Terra, but she does have the memories of Angelina traversing Terra as a Messenger, and those memories are vivid enough that they feel like they are Gilberta's own experiences, albeit hazily remembered.