In general, when an application closes/ends the storage it occupied is instantly released, and if you restart the application it will be loaded from disk afresh, into "new" storage.
The biggest exception to this would be on some "smart phones", where sometimes when you "end" an application it's not really ended immediately but is simply "backgrounded" for a period of time on the assumption that you may restart it in the near future. But if you don't use the application for a period of time it's eventually ended by the OS and the storage reclaimed. (So far as I know this scheme is not used on regular Microsoft or Apple computers, only phones.)
Another exception, a bit more technical, is that even though the program is ended and its storage released, the disk image of the program (or at least parts of it) may still exist in disk "cache", and may be more easily accessed a second time than the first time. This may be the cause for what you perceive as a more rapid startup the second time.
There is another, simpler reason for that faster startup, though. When you end an application and then immediately restart it there is this convenient "hole" in storage that exactly fits your program. If you start it later, after running other things, the OS has to muck around finding available storage, and this often means that it must "page out" other data in RAM, a time-consuming process.