Finger cracking comes from fluid between joints
The sound of cracking fingers does not come from bones or tendons. In fact, doctors didn't discover how the sound is made until 2015!
1 The fingers are joined via a joint capsule filled with lubricating fluid. The joint fluid is full of dissolved air molecules such as nitrogen and carbon dioxide. 2 When we crack our fingers, the joints are pulled apart, and the joint capsule volume increases, causing an empty space in the fluid, into which the air molecules rush, causing the cracking sound.
BONE JOINT FLUID JOINT CAPSULE AIR MOLECULES