I understand at a both a very high and very low level how LLMs are trained - can someone here help me better understand the middle? I understand how one could

I understand at a both a very high and very low level how LLMs are trained - can someone here help me better understand the middle? I understand how one could build a training set of CVs, job descriptions, and outcomes. How much data would be needed here to create training and validation sets large enough to influence and confirm adequate performance?