There are no single answers to either of your posts, but back squats are the biggest developer of leg strength and ergo increase the capability of exerting force.
At top speed elastic strength is critical to minimize ground contact time. The support phase is important at top speed as well to maintain the “sprint” position.
Top speed work, plyos, and solid core work are very important elements for reaching and maintaining top speed.
I would rank squats and other weightlifting exercises more important in terms of starting strength and acceleration. Overcoming the initial inertia of the start.
As running speed increases, ground contact time decreases. The limiting factor therefore shifts from maximum force (i.e. strength) over the first 20 metres, to rate of force development at top speed. Of course increasing maximum strength [i]can[i] also have positive benefits on RFD.
Bill is correct, elastic recoil from tendons is a substantial contributor to force applied at top speed. Muscles contract on (before!) ground contact. The contractile component must attain a tension sufficiently high that it is ‘stiffer’ than the attached tendon. The faster this threshold tension is achieved (the higher RFD!) the more energy is stored in the tendon. Current work by my supervisor - Tony Blazevich is investigating the optimum compliance of specific tendons for sprinting (and jumping). Unfortunately, [i]length[i] of tendons is genetically predetermined.