What are open source projects? Are they free-to-use-without-any-credit like public domain characters such as PInocchio or Winnie-the-Pooh?

Many projects written in languages such as C or Zig are compiled. What this means is that human-readable code gets translated into machine code. There is no “decompiler” for these languages, so people can't modify them.

If a program is open source, then the source code (human-readable code) is open to the public. This does not necessarily mean that you can do anything with it. If you see a license, that's basically giving you permission to do certain things with the code. Usually it's something like “you can do whatever you want as long as you include this license and you credit the original person”.

If the program doesn't have a license, then legally it's copyrighted and you can't modify or distribute it without the creator's permission.

Open source projects are projects that grant you the four freedoms as they are called. Note how open source is also called free software, the definition of this is “free as in free speech, not as in free beer”. Not all gratis (zero cost) software is open source, for example Roblox is not open source and called “freeware” which is very different to free software.

Freedom 0: The freedom to use the program for any purpose.
Freedom 1: The freedom to study how the program works, and change it to make it do what you wish.
Freedom 2: The freedom to redistribute and make copies so you can help your neighbor.
Freedom 3: The freedom to improve the program, and release your improvements (and modified versions in general) to the public, so that the whole community benefits.

Basically, this requires developers to post their code somewhere, usually online, and to license their software under a free licence. Software can be copyrighted as any other work, and if it does not carry a licence, it is considered All Rights Reserved (look but don't touch). This is why all open source projects have a licence.

For example, Linux, Scratch, Firefox, GIMP, MediaWiki (the software used by Wikipedia) and all shared Scratch projects are open source. Because of this, your car, TV, robot vaccum, phone and router probably run some version of Linux. Open source can be commercial, for example, SUSE is a popular enterprise, commercial Linux distribution.

Windows, Google Maps, Minecraft and ChatGPT are not open source, commonly called proprietary or closed-source software. For example, it is not legal to modify Windows, call it something else and sell it on DVDs.