They deploy the models with well established pipelines. Usually (from my experience), scientists communicate closely with engineers (ML, Data Engineers or DevOps). They "put" the model into the environment where it can operate. It can be either the web service where u simply wrap the model, or container deployments (see Docker). They don't retrain the models every-time the app launches as they simply save the models parameters in that environment or CI/CD platform. I have a little experience in deploying and maintaining the production models, but I have no clue about the details tho.