How summoning works in lore has been an extremely vague and controversial topic almost as long as Magic: the Gathering has existed.

Early on, you summon a creature, it was bound to you. Summoning legends required considerably more work, since they weren't ordinary creatures.

At some point, the idea of you summoned a 'copy' came into fashion, and WotC jumped between both idea's numerous times, even as late as the webcomics, where Garruk can clearly be seen branding his creatures to him.

Brady Dummermouth once did an AMA on the mothership forums (almost 20 years ago now) and when I asked him, he said it was intentionally being kept vague because not being able to summon actual living things takes a great deal of excitement out of the story while the other option is just pure slavery.