Like for when I want to start freelancing and earn money, what sites/programs could I use to build projects for them to be able to use? Could I use codepen.io for that?
The answer really depends on what the client expects of you and what you focus is.
For example, if the client doesn’t care, and will actually pay you to manage a simple site for them, you could use almost anything that covers the project requirements. Something like codepen will show a codepen bar at the top and might not provide the best SEO for the site, so it probably could never be used for a production grade project.
You could manually set everything up on a VPS like Digital Ocean, use static hosting like netlify using the JAMStack. Or use some other hosting solution out there that runs cheap/free.
If you require a back-end+database, other features you might need a full on cloud solution like Amazon Web Services.
Or maybe all of this is way to development much work and you just want to use something like wix.com to build a simple site using drag and drop tools, along with provide hosting.
There are a number of solutions out there, each with their pros and cons. Understand your client’s requirements, and select the right one for the job. If you aren’t sure what the “right” one would be, then I’d review all the different options to get a better understanding of what options are available for you to learn and offer to clients.
Finally, its worth mentioning that freelancing itself requires a lot of other work beyond just “doing the work”. Id recommend looking more into what it takes to freelance successfully.
Good luck, keep building, keep learning!