The assemblies in a sandboxed solution are included in the solution package (.wsp file), and the package is deployed to the site collection's Solutions Gallery. When a sandboxed solution is accessed for the first time, such as when a user navigates to a page that contains a Web Part from a sandboxed solution, any assemblies in the solution are extracted from the package in the gallery and copied to the file system of the server that is handling the sandboxed request. The location is C:\ProgramData\Microsoft\SharePoint\UCCache. The executable of this service is SPUCHostService.exe. The server that handles the sandboxed request is not necessarily the front-end web server that is handling the initial HTTP request: The Microsoft SharePoint Foundation Sandboxed Code Service can be run on back-end application servers in the farm instead. Because the sandboxed user process (SPUCWorkerProcess.exe) cannot copy anything to the file system, the copying is done by the Microsoft SharePoint Foundation Sandboxed Code Service.
The assemblies of a sandboxed solution do not stay in the file cache perpetually. When the user session that accessed the solution ends, the assemblies stay in the cache for only a short time, and they may be reloaded from there if another user session accesses them. Eventually, if they are not accessed, they are removed in accordance with a proprietary algorithm that takes into account how busy the server is and how much time has gone by since the assemblies were last accessed. If the sandboxed solution is used after that time, the assemblies are extracted again and copied to the UCCache.