4. nlohmann-json3-dev is available in the official repo for Ubuntu 20 and up, e.g. https://packages.ubuntu.com/focal/nlohmann-json3-dev
Since dcap deprecated support for Ubuntu 18, the ppa is not needed. As it is best practice not to have unneeded PPAs, I suggest removing this from the docs.
src/Linux/README.MD 9:*
sudo add-apt-repository ppa:team-xbmc/ppa -y