What does copyleft require?
Copyleft requires that modified and redistributed versions of a program stay under the same free terms. The Free Software Foundation describes it as making a program free "and requiring all modified and extended versions of the program to be free as well" (GNU, What is Copyleft?). GPL version 3, dated 29 June 2007, defines "convey" as propagation "that enables other parties to make or receive copies"; you may make, run and propagate covered works you do not convey "without conditions so long as your license otherwise remains in force" (section 2), but a modified work you do convey must be licensed "as a whole" under the GPL (section 5(c)) (GNU GPL v3). The AGPL adds a duty to offer source code to users who interact with a modified version "remotely through a computer network" (GNU AGPL v3, section 13).
Why does copyleft matter when you license a codebase?
Most company codebases mix code you wrote with open-source dependencies, and your company can only license what it holds the rights to. Handing a licensee copies of GPL-covered files lets them receive copies, and section 4 of the GPL then requires keeping its notices intact and giving recipients a copy of the licence (GNU GPL v3, section 4). Licence terms can pull the other way: micro1's published data licence terms ask the licensor to warrant that the licensee will not need to "give attribution or other permission to any third party" (micro1, Data License Master Terms, section 5.2). The GPL v3 text does not mention model training, so how its conditions apply to training data is a question for your counsel.
What does this mean for what you license and control?
The cleanest scope is usually the code your company wrote, with third-party code identified and either excluded or listed with its licence. Your company can keep ownership of its data. This is agreed case by case. This page is general information, not legal advice.