I did not watch the YouTube video, but based on your description, it seems to be presenting a garbled account of history.
Constantine XI did not will his titles way, but his brother Thomas Palaiologos claimed the imperial title after his death. Thomas briefly ruled the Byzantine remnants in the Peloponnese, but fled to Italy ahead of an Ottoman invasion in 1461. After his own death in 1465, Thomas' claims were inherited by his eldest son Andreas.
Andreas was the one who willed the imperial titles to Ferdinand and Isabella.
On April 7, 1502, Andreas executed his testament designating Ferdinand and Isabel and their successors as his universal heirs.
Freiberg, Jack. Bramante's Tempietto, the Roman Renaissance, and the Spanish Crown. Cambridge University Press, 2014.
This came after he sold the very same titles to Charles VIII of France, though, so the Spanish monarchs appears to have never used them.