Temporally, we are chosen "in Christ" when we believe and are regenerated.
At that moment: Spiritually, we are raised up and made to "sit together in the heavenlies in Christ Jesus" Who is not confined
by temporal space and time. Thus, allowing Eph 1:4 to be true for us, only when we are "in Christ".
Eph 2:5 (even when we were dead in sins) has made us alive together with Christ (by grace you are saved),
Eph 2:6 and has raised us up together and made us sit together in the heavenlies in Christ Jesus,
So again what does “BEFORE the foundation” mean in your understanding?
“even as he chose us in him before the foundation of the world, that we should be holy and blameless before him. In love” (Eph 1:4)