John Gill Commentary Mark 6:46

"And after he had taken leave of them, he departed into the mountain to pray." — Mark 6:46 (ASV)

And when he had sent them away
Either his disciples, or rather the multitude:

he departed into a mountain to pray :
after his disciples were gone, and he had dismissed the people, he went from the desert where he had been, up into a mountain; being a retired place, to spend some time in private prayer to God; (See Gill on Matthew 14:23).