A shortcut acts in the current app or focused area. Check the insertion point or selection first. Common Windows text shortcuts use Ctrl with A, C, X, V and Z for Select all, Copy, Cut, Paste and Undo. Mac text editing commonly uses Command with those letters. Menus or touch controls are valid alternatives; check the app for differences and Redo.
Worked example
The message field is active, but you mean to replace only a title. Activate the title first, then select the intended text. A shortcut alone does not know your goal.
The Fictional Poster Editor
The editor has two editable text boxes: Title contains SKY and Note contains Bring paper. Note currently has focus. In this model, Select all selects the contents of the focused text box only. You intend to replace the Title with SPACE while keeping Note unchanged. The app offers labeled edit-menu commands as well as keyboard shortcuts. Do not practice on passwords, messages or important files.