Is there a way to specify a project to be opened automatically every time I open the software?
I’m not sure if it is what you are looking for, but the “Reopen projects that were open on quit” option in the General prefs tab will do what it sounds like. So, it doesn’t quite open a specific project you declare, every time, and may open other projects along with it.
What I would do is just start Scrivener via that project itself. That is how I generally launch the software, by opening the project I intend to work on directly via the OS. If it’s the one you always want, try dragging that project icon to the Dock and removing the general Scrivener application icon.