Code::Blocks is a very good free integrated development environment (IDE) for writing in C, C++ and Fortran. It has been around for a long time and is recommended as the learning environment on sites such as Learn C++.
To install Code::Blocks on Kubuntu 20.04 open up Konsole and type,
sudo apt install codeblocks build-essential
The build-essential package may already be installed but is required if it is not.
Code::Blocks can be found under the Applications Menu at Applications/Development. After starting the IDE for the first time it will ask you which compiler you wish to use. If build-essential has been installed then the GNU GCC compiler will be available.