How did Guru Nanak Dev Ji “meet” Akal Purakh if Akal Purakh is formless?

Since Akal Purakh (Waheguru) is described as Nirankar (formless), how did Guru Nanak Dev Ji have a “meeting” or “conversation” with Akal Purakh during the Bein River incident?

Guru Nanak Dev Ji disappeared for three days and then spoke about receiving a divine message? If God has no physical form, what exactly happened during this experience? Was it a spiritual realization or enlightenment rather than a physical encounter?

I’d really appreciate explanations based on Gurbani or Sikh teachings.