If you don't manage that team and "the thing" is working, you need to define what is the problem you are trying to solve and why. You are describing a software

If you don't manage that team and "the thing" is working, you need to define what is the problem you are trying to solve and why. You are describing a software that is written in a naive or obsolete way, but other than doing development in production there is no critical problem that you can fix to bring immediate value. I saw this in the past a few times. There is no universal recipe, if this is what you are looking for. Get some development and stage environment and make them use Git, that's a start. See what is the plan for that software, maybe the company does not want (you) to waste time and money with it, if they want to do something, discuss and align that. In the end, if it works it brings value. If you want to rewrite it, it will bring some value and some cost: which is bigger and what is the priority, a rewrite or new features? One more thing you can do it show the developers how to do some things in a better way, like composer or cleaning up versions and dependencies, but take it easy and present it to them in a way they will buy it and do it themselves, not because you told them so. Make them better and they will make the product better.