You will need to pass in the id for the Technicien you want (usually as a URI segment):
public function GetTables($id)
{
$technicien = Technicien::with('user')->findOrFail($id);
return [
'id' => $technicien->id,
'actif' => $technicien->actif,
'moyenne_avis' => $technicien->moyenne_avis,
'nom' => $technicien->user->nom,
'prenom' => $technicien->user->prenom,
'adresse' => $technicien->user->adresse,
];
}