How is the heart's reliance on God achieved in reliance (tawakkul), and does the word "حسبه" (hasbuh) in the verse: {And whoever relies upon Allah - then He is sufficient for him} mean a promise from Allah to achieve the desired outcome of reliance, such as success in studies or business?

What helps you achieve perfect reliance on Allah is the knowledge that all matters are in His Hands; there is none to withhold what He gives, and none to give what He withholds. He alone possesses the power to bring benefit and ward off harm, and He is over all things competent. He is the Self-Sufficient, the Most Merciful, the Most Wise. When you keep these meanings in mind, your reliance on Allah will be complete.

Reliance (tawakkul) does not negate taking means (asbab). Reliance is an act of the heart, while taking means is an act of the limbs. Therefore, you must take the required means, then attach your heart to Allah, trusting in Him and depending on Him.