even tiny roblox games make money. i developed for a small game (~10m plays) a few years back and i still earn a decent residual from player revenue, even as the game slowly loses traction
I still got enough robux to DevEx just from premium roblox subscribers playing the game. (If I cash it out i would get a few hundred dollars. Which is nothing for me today but absolutely a lot of money for an aspiring teenage developer.)
If we are talking about game mechanics, they get copied all the time, of course. If it's about superficially reproducing a game including the assets, I don't think any big players on PC or consoles steal ideas from indie/smaller developers. That is despised and likely not allowed by the platform, if not challenged in court.
(I have no idea about what happens on mobile platforms)
The platform doesn't care when someone makes a copy, there's no way for players to push back and actually hurt the developer making the copy (unlike Steam where trying to do it will get you bombarded with negative reviews and buried by the algorithm), and superficial visual quality and marketing attracts players more than actual gameplay quality.
my advice is to just start building something. the Toolbox has thousands of 3D models which make the initial experience easier and more fun, and there are hundreds of YouTube tutorials teaching you the basics of Roblox scripting.
also use the excellent DevForum, which has thousands of explainer posts:
https://devforum.roblox.com/c/resources/community-tutorials/...
edit: looked at the old stats online. a few years back that game was pulling $1,500+ with no effort each month