Looking at their installer it should've told you how to use it after the installation.
Relevant snippet:
if [ "${custom_dir}" == "false" ]; then
output "\nUse it as a local file:" "info"
output " ${binary_dest}/${CLI_EXECUTABLE}"
output "\nOr add the following line to your shell configuration file:" "info"
output " export PATH=\"\$HOME/${CLI_CONFIG_DIR}/bin:\$PATH\""
output "\nOr install it globally on your system:" "info"
output " mv ${binary_dest}/${CLI_EXECUTABLE} /usr/local/bin/${CLI_EXECUTABLE}"
output "\nThen start a new shell and run '${CLI_EXECUTABLE}'" "info"
fi
If you haven't provided a custom directory it should be installed in ~/.symfony5/bin/symfony. As the installer should've printed out you can either execute that binary directly, add export PATH="$HOME/.symfony5/bin:\$PATH\" to your shell configuration file (e.g.~/.bashrc) or move it to your system's /usr/local/bin folder. After that you can simply launch it using the symfony command.
