Dojos are areas in the Adventure Mode: World of Light map that are unlocked through getting the Master Spirit that leads the Dojo. Dojos can be accessed by going to the house where the training is done or through the Activities tab in the Pause menu.
Once the Master Spirit has been achieved, they will go to their home and offer a training service for your spirits. Up to four spirits can be in most Dojos at once. Upon entering, you can select one of the four slots. After selecting said slot, you can select what primary spirit you want to put there. If you just leave the spirit there, it'll slowly learn the style in question. You can remove the spirit at any time. Once the training is complete, the spirit will incorporate the style and be removed from the slot. However, there's a way to speed it up: after a spirit is in a slot, upon selecting that slot again, the Dojo master will prompt you to spend 500 Spirit Points to learn the Dojo's style immediately. The training, in any case, does not leave the spirit exhausted, unlike the Explore and Gym functions. Once the training is complete, the spirit will be removed from the slot. Spirits can only know one style at a time. The style the spirit is currently using can be seen by pressing the X button while hovering over that spirit in the spirit selection menu for the Level Up, Create Team and Dismiss functions.
Every Dojo has its own style. Every style enhances the spirit's abilities in one aspect but hinders them in another (for example, Ground Style increases the player's movement speed and damage on the ground but decreases their midair damage if their primary spirit is using that style). Slowpoke's Dojo allows spirits to forget the style they learned. This one only has one slot, and any fighter put there will immediately lose their style, without a prompt to see if the player is sure about it or not. That spirit can then be trained again at any time. Training a spirit that already knows a certain style will replace the old style with the new one. A spirit cannot train to learn the style they're already using.
Once a spirit has known a style once but they are not using it anymore (through Slowpoke's Dojo or through learning another style) training them to learn that same style will mean they'll remember it and thus re-learn it instantly. You can also see what styles your spirit has learnt in the same way that you can see their current style.