How to set default app for .ps1 files to PowerShell 7 installed with MSIX installer? #27868

You can't, it requires the msix manifest to be updated. See #27277

The default-app picker can't do it — the MSIX manifest doesn't declare a windows.fileTypeAssociation (see #27277 / PR #27875), and C:\Program Files\WindowsApps is locked down on purpose, so don't fight that folder. You don't need the real exe path anyway: use the app execution alias, which lives in a user-writable location and is stable across package updates:

%LOCALAPPDATA%\Microsoft\WindowsApps\Microsoft.PowerShell_8wekyb3d8bbwe\pwsh.exe

(The per-package-family subfolder is better than the generic ...\WindowsApps\pwsh.exe because it still exists if the alias is disabled or collides with another pwsh.) Explorer's "Open with" dialog refuses aliases, so set it in the registry. .ps1 maps to the Microsoft.PowerShellScript.1 ProgID, and a per-user override of its Open verb takes precedence over the machine default (Notepad):

$pwsh = "$env:LOCALAPPDATA\Microsoft\WindowsApps\Microsoft.PowerShell_8wekyb3d8bbwe\pwsh.exe"
$key  = 'HKCU:\Software\Classes\Microsoft.PowerShellScript.1\Shell\Open\Command'
New-Item -Path $key -Force | Out-Null
Set-ItemProperty -Path $key -Name '(Default)' -Value "`"$pwsh`" -NoExit -File `"%1`""

Double-clicking a .ps1 now runs it in PowerShell 7 and leaves the window open (-NoExit). Drop -NoExit if you want it to close, add -ExecutionPolicy Bypass if your policy blocks local scripts. No admin rights needed, and it survives pwsh updates because the alias path never changes. For the preview package the family is Microsoft.PowerShellPreview_8wekyb3d8bbwe.

Two caveats:

• If you ever picked a default app for .ps1 via "Open with → Always", Explorer uses HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.ps1\UserChoice instead of the ProgID. Delete that key (reg delete "HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.ps1\UserChoice" /f) and the override applies.
• If you'd rather keep double-click = edit (the reason Microsoft never shipped this association), put the same command under HKCU:\Software\Classes\SystemFileAssociations\.ps1\Shell\RunWithPwsh7\Command and set the RunWithPwsh7 key's default value to Run with PowerShell 7 — you get a right-click verb instead.