For Joomla I create custom applications like extensions, templates, components, modules or plugins.
I have already done a lot, but not all submitted to Joomla. Soon there will be a lot of information here, including how to create extensions, tips and tricks from me.
Why publish extension ?
So as a developer, you develop a lot over time and do not publish it because it's not legal. Either you have developed it for the team or the code is legally subject to the company. But since you can develop as a developer for yourself and can grant your own license you should publish it.
I have developed so much and these are running somewhere, but no one knows about it so publishing. And preferably directly into the repos of the respective CMS, because if you need it again you can use it yourself (especially as a developer).