If you worked on this while employed without getting a waiver their lawyers might beg to differ in the UK pretty much every contract I’ve seen has “we own all your discoveries unless stated otherwise” clause in it.
"In the course of your employment" or similar words used to be a more reasonable standard that was widely considered fair to both sides for salaried employees. In other words, things you do on company time, using company resources and/or in the company's line of business go to the company, but they have no claim beyond that. I haven't spoken to a lawyer about this recently, so if the issue might affect you, please check whether this is still correct and don't just rely on my comment here.
And what if you actually hire people to work on it for the time after you join the other company?
Where does it end?
What if your parents never met?
Please don't read this as facetious, I'm not meaning to be.
What I am asking is what happens with work/projects/apps that someone has done on the side before joining a company, and especially in the case that this creates a revenue stream.
In my current role I mentioned that I work on things on the side and they were like... oh that's not allowed, and I said... oh well I can't work here then.
They changed the rule lol
Good for you! There seems to be a certain mindset in some company bosses, and it's usually middle management types who have made it to a moderately senior level and never not been "company men" (or women etc. obviously) in my experience. It's like they think becoming an employee at their firm means giving over your life to them, and that by graciously allowing you to work for them they're doing you some big favour that justifies that stance. This is supported by the corporate lawyers routinely trying to insert heavily one-sided terms into employment contracts. But that simply isn't what an employment relationship is, certainly not in the West in 2020, and enough good people saying "no" and being willing to walk away is the only way the "we own you" people will learn.
When I interview for a position I am interviewing them as much as they are interviewing me. Everyone should think like this.