On this project, I configured .vscode/settings.json with "workbench.startupEditor": "welcomePage".
I created a welcome.md file in the project root.
But when I open this project in codespaces, I see the readme, not the welcome page.
First of all, this settings seems to have no effect at all in Codespaces. Codespaces ALLWAYS opens a readme if available.
Second, if it worked as specified, it would not do what you think it should.
This setting is not meant to open an arbitrary file of your own except a readme.
Accepted Values are only none, welcomePage, readme, newUntitledFile and welcomePageInEmptyWorkbench.
And it's only meant to show the specified Editor if there is no other editor window to be restored from the last session.
Readme is the only selection to show an Editor with content of your own.
welcomePage will NOT show any file named "welcome" by yourself but the Visual Studio Welcome Page, which is the one that you get by selecting [Help][Get Started] from the menu.