These allies are summoned through artifacts. These artifacts are unlocked in 3 main ways: through the reward chest for completing a level, by purchasing a random artifact from the wandering trader (joins your camp early in the Default playthrough), and through the random set drops for each level (these can be viewed from the level select map, but don't guarantee that the specific item will drop in your playthrough). The artifacts you need to unlock are the Wonderful Wheat (Llama), Tasty Bone (Wolf), and Golem Kit (Iron Golem). While the first two can be unlocked at any point thanks to RNG, the Golem Kit only becomes available as an unlock after completing the game on both Default and Adventurer difficulty.
Here is a playlist of guides for this game
https://www.youtube.com/playlist?list=PL-HeW6XkKdUKsFThxuvCp...