Moving personal notes ("second brain") between companies

Moving personal notes ("second brain") between companies When an employee at a particular company has built a personal knowledge base (often called a "second brain"), what happens when they move to a new company? For example: Joe, an employee at Company A, makes a personal knowledge base using OneNote through the company's Microsoft subscription. A few years later, Joe decides to move to Company B. What happens to that collection of knowledge? Would Joe be allowed to export those notes and use them at Company B? That seems questionable since (I think) the notes would be considered a work product belonging to Company A. The notes might even contain confidential information about Company A's systems that Joe worked on. Otherwise, could Joe have instead developed the second brain on a personal account that he logged into using Company A's computer, then continue to use this at Company B? This also seems iffy. I think the work product idea might still apply, plus, in security-conscious environments, it might be questionable for Joe to be logging into personal accounts on the company computer. (I'm looking at defense work after I graduate, so I expect this to be an issue.) Else, does Joe just rebuild everything every time he moves companies? Is this perhaps less of a big deal than I'm assuming? For context, I'm a grad student interested in the potential benefits of a better organizational system for notes, such as a second brain. However, since I may work in the defense industry after I graduate, I keep getting hung up on the concern that whatever systems and habits I develop right now, I will have to discard/rebuild in the future when I transition between organizations.