in terms of solvers i no of 3: single chain (SC), rotate plane (RP) and IK spline

the single chain is for connecting joints of 2 bones usually, and its ur basic IK handle

the rotate plane is fun, its the same as the SC but gives you a pole vector control. so for example you have a bent elbow, the pole vector would allow you to control the direction the elbow points

the ik spline is for spines of characters (there are some rules you have to follow wen creatin ik splines tho). this solver use the CVs of a EP curve to move bones in the backbone