The context is the building of the Tabernacle in the wilderness, and the subject is "spirit." Spirit is what impels and motivates.
We need to understand the structure of verse 21. The second phrase, "and every one whom his spirit made willing" is parallel with the first phrase, "they came, every one whose heart stirred him up." Parallelism is a Hebraic syntactical form used to clarify and reinforce a statement.
The human heart, the blood-pumping muscle, is invisible to sight, being inside the body, but it is the activating and animating source of physical life: "The life is in the blood." The Hebrews knew that, and so they used "heart" in a way that parallels "spirit." In their speaking and writing, Hebrews used the heart to symbolize the seat of the intellect, as well as the emotional and spiritual life. To them, it was also, not only the animating part of physical life, but also the invisible energizer and motivator of activity in the areas of intellectual, emotional, and spiritual life. They simply made a connection between the physical heart and its energizing physical life, and spirit, which energizes intellectual, emotional, and spiritual life.
Thus, in this verse, "heart" and "spirit" are brought together as though they were one, involved in stirring, exciting, or motivating people into action to build the Tabernacle. We could take this one step further to say that spirit was exciting or inclining the mind to be willing to choose to give of themselves.
John W. Ritenbaugh
The Holy Spirit and the Trinity (Part Three)