How do I save all the tabs as one PSD in Photoshop 2020?

If you wish to combine all open tabbed documents into a single file, then use the Load Files Into Stack script as suggested by Marianne-Deiters.

If the files are a smart object layer, such as opening a file from ACR as a Smart Object, then you will need a custom script, such as found here:

https://community.adobe.com/t5/photoshop/how-to-auto-stack-smart-objects-into-single-file/m-p/10833135#M294490

If you did wish to save all open tabbed documents to separate files, then a custom script will be required. A file format with appropriate options would need to be coded, such as PSD with layers, alpha channels and ICC colour profiles.

If you are happy to have the Save As dialog box pop up you could insert the Save As command into an action, then reference the action in a script such as this:

for (var i = 0; i < app.documents.length; i++) {
    app.activeDocument = app.documents[i];
    // Change the action and action set name to match your action!
    app.doAction("Save As Action", "My Save As Action Set.atn");
}
while (app.documents.length) {
    app.activeDocument.close(SaveOptions.DONOTSAVECHANGES);
}

Choose File >  Scripts > Load files into stack - klick on ADD OPEN FILES and OK - see screenshot

The tabs are separate documents. There is no way to just save them as one without any further intervention. You proactively have to insert the contents of the documents into other documents using copy&paste, linking smart objects, drag&dropping files onto a document and what have you.

Mylenium