Pirat the best would be to do that via an extension as already said in the other discussions.
Though if you really want to work on your own core modification, I suggest you edit javascript source files (in js/forum/src and use the Gulp command (in beta7 there's a script in the script folder I believe, you'll need npm for it to work) to regenerate the dist file. This will allow you to use the JSX syntax and all the latest javascript (is it es6, es2016, I never know) methods. This will also make it a lot easier to manage conflicts when you'll merge new core versions.
If you have properly created your own core copy of core outside of vendor (in a workbench folder ideally, and it doesn't look like you did from your last post), then editing dist files isn't technically wrong and should work and not be automatically wiped out. Though it will likely become complicated to maintain for the reasons already mentioned 🤷♂️