Using an online compiler is not the best way to learn C++ since many key features are not easily available such as reading/writing files, interaction with the user, etc. As Peter87 suggests getting a compiler/IDE installed on your computer might be a better use of resources.
Depending on your OS there are a couple of IDEs I could recommend: Visual Studio 2019/2022 Community or Code::Blocks.
https://visualstudio.microsoft.com/downloads/
https://www.codeblocks.org/
Both allow for source editing, compilation, execution and debugging from a common app.
VS 2022 requires 64-bit Windows, 2019 can be installed on 32-bit.
Both are free. VS can be a major HD space hog, but for Windows has the best debugger available. If your OS is Mac or *nix Code::Blocks is a good choice. Though there are other compilers/IDEs available.
Now, if you want to do old school command-line compiling both VS and C::B offer tools to do that. A third command-line compiler is MSYS2.
https://www.msys2.org/
I have all three installed -- Visual Studio 2019 & 2022 (Community), Code::Blocks and MSYS2 -- it never hurts to have 2 or more different compiler suites to make sure code you write is as agnostic as possible.
I doubt it will make a huge difference but currently VS is the only compiler/IDE that is fully compliant with C++20. That matters to me.
Should you get VS C++ is NOT installed as part of the default packages, you have select it (desktop development with C++) manually.
A good resource that is free to learn C++ is Learn C++:
https://www.learncpp.com/