What entitlement actually is
The VA does not lend money. It guarantees a portion of a loan made by a lender, and entitlement is the amount of that guaranty available to you. Understanding it as a guaranty rather than as a loan explains most of what follows, including why using it once doesn’t necessarily consume it forever.
Your Certificate of Eligibility is the document that establishes what you have. It’s the first thing to pull, and it can be pulled before you look at a single house.