Advanced users with special needs can download the Signal APK directly. Most users should not do this under normal circumstances.
You can verify the signing certificate on the APK matches this SHA256 fingerprint:
29:F3:4E:5F:27:F2:11:B4:24:BC:5B:F9:D6:71:62:C0 EA:FB:A2:DA:35:AF:35:C1:64:16:FC:44:62:76:BA:26