How to download and setup runtime
Open terminal and run command
                                            git clone https://github.com/symfony/runtime.git
                                        
                                        git clone is used to create a copy or clone of runtime repositories.
                                        You pass git clone a repository URL. it supports a few different network protocols and corresponding URL formats.
Also you may download zip file with runtime https://github.com/symfony/runtime/archive/master.zip
Or simply clone runtime with SSH
                                            [email protected]:symfony/runtime.git                                    
                                    If you have some problems with runtime
You may open issue on runtime support forum (system) here: https://github.com/symfony/runtime/issuesSimilar to runtime repositories
Here you may see runtime alternatives and analogs
node symfony grav ilios ChakraCore dragula rome horsey insignia jkanban Croppr.js EasyAdminBundle NelmioApiDocBundle SensioFrameworkExtraBundle LexikJWTAuthenticationBundle VichUploaderBundle DoctrineMigrationsBundle framework-bundle LiipImagineBundle FOSElasticaBundle SncRedisBundle KnpMenuBundle twig-bundle swiftmailer-bundle security-bundle web-profiler-bundle NelmioCorsBundle CraueFormFlowBundle SensioGeneratorBundle web-server-bundle