It runs in user mode rather than as a system service. It only starts when an application connects to it and shuts down when not in use.
While it doesn't require a complex setup, it still requires a one-time LocalDB MSI installation (~70MB) on the host machine. 2. The True Portable Alternatives ms sql server express portable
If you absolutely cannot install anything on the host computer, you should consider a database that is designed to be truly portable (zero-install). Portable SQL Server - Server Fault It runs in user mode rather than as a system service
