How do I understand the law behind HR policies?

You follow the policies faithfully, but you do not really understand the law they come from, and that gap makes you uneasy. That unease is actually a strength. Plenty of people follow rules without ever asking why. You want to understand, and that instinct is what turns a policy-follower into a real professional. Every policy sits on top of a law or a risk it is meant to manage. When you learn the why behind a rule, two things happen: you remember it better, and you can handle the situations the policy did not quite anticipate. You do not need a law degree. You need to get curious about the reason behind the rules you already apply, one at a time, until the logic underneath starts to show. Pick one policy you enforce often and find out what law or risk it exists to address. Just one. Do that regularly and, over time, you stop memorizing rules and start understanding a system. That understanding is what makes you genuinely good at this.