Buy and install the eSIM while you are still home on your own wifi. You get a QR code by email after you order. Open your phone settings, choose to add a mobile or cellular plan, then scan that code. The whole thing takes a couple of minutes. You do not turn the data on yet. You just load it so it is ready.
First check your phone can use an eSIM. Most iPhones from the XR onward work, plus Google Pixel and recent Samsung Galaxy phones. The phone also has to be carrier unlocked. If you bought it on a contract, ask your provider to confirm it is unlocked before you travel.
Pick your plan size now too. For a short city break in Chisinau where you mostly use maps and chat, the 3 GB for 15 days plan at $2.10 is plenty. If you only need a quick top up for a single day, the 1 GB for 1 day plan starts at $0.76.