Open source (or open code) are software whose codebase is open to the public, and kept by institutions or individuals. That implies all open source projects have their recipe exposed in plain sight for anyone to access, fork and mess around with, unlike software-as-a-service (such as Adobe Cloud) or private code that belongs to companies, banks and governments.
Some open source projects accept pull requests, which is the act of downloading the code of a project, building on top of it - either by adding new features, rewriting snippets or fixing bugs - and submitting those contritbutions to the mantainers of that project, which in turn will check your work and decide whether or not they'll accept your request to collaborate, or not.
There's a huge community of open source projects and contributors that's almost as big as the internet itself, and all of this is possible thanks to the strong culture of collaborating that exists in the developer community ever since it became a thing. Developers will spend hours on a row reading, writing and teaching code for free, which is a rare sight in a world that's getting more transactional by the hour.
Most programmers I know have learned a great deal from strangers on the internet. Eventually their chance to teach and give back comes, but even the most senior developer will still ask for help sometime.
There are companies, both private and third-sector, that employ developers and designers to work professionally in open-source projects. Some examples are Mozilla, Rocket Chat and DuckuckGo.
Some companies even start as open source projects, and then start operating as SaaS. Ghost, Elastic and Vercel are some of the services that were once open source.
There goes a list of open source software, what they do, and what private software they can be used in the stead:
Krita, digital painting and drawing (Photoshop)
Gimp, image editing and digital painting (Photoshop)
Inkscape, vector-based image editing (Illustrator)
Firefox, the famous
Thunderbird, emails (Outlook)
DuckDuckGo, search engine (Google)
Chromium, the open source to Google Chrome
Minetest (Minecraft)
The same way programmers tinker with open source code for their own learning and self-improvement, designers might use open source to practice drawing assets and screens. Some projects desperately need a designer's touch! 🥲
The fluidity in open source allows for a project to be duplicated, line by line, and later hosted somewhere else to be marketed as a proprietary software product or service. That's not a rare sight and I've seen a dozen startups that began their companies this way.
It's a good practice to give credit when using any code written by somebody else, except if they explicitly state there's no need to.