With most packages, the clock starts neither at purchase nor at installation; it begins once the eSIM picks up a network inside the country it covers. That means you can set it up in Saudi Arabia ahead of your trip and lose no days. A few packages are different and start counting the moment they are installed. The package details say so both before and after you buy, so give them a read.
Use a stable connection, preferably Wi-Fi, so the whole eSIM downloads. There are three ways to install: straight from the app, with a QR code scan, or by typing the details in yourself. If you install from the app, it asks where you are: pick Get Connected if you have already arrived, or Connect Later if you have not. Stay on the screens until activation finishes.
Once it is installed, switch the line on and choose it for mobile data, then adjust data roaming for the eSIM as instructed in the app. When installation fails and the screen shows “Something went wrong”, the View options button leads to replacement, refund and support choices.
Source: Airalo help: Installing and setting up an eSIM on iPhone