Settings → Connections → SIM manager → Add eSIM, then scan the QR code from your order page. Some Galaxy models word the last step "Scan QR code from service provider".
If scanning is not offered or fails, choose to enter the details instead and paste the whole activation key from the order page.
Keep the phone on wifi while it installs, and install before you leave rather than at the airport.
Settings → Network & internet → SIMs → Add eSIM, then scan the QR code on your order page. The wording differs a little between makers, but the path is close on all of them.
Most Android phones also offer to type the details in. They give one field and want the whole activation key, which you can copy from the order page.
Install on wifi, before you travel, and leave the eSIM in place once it is there.
Every order page carries the same details as text. Most Android phones give you one field and want the whole activation key: copy it from the order page and paste it in.
Some phones ask for two separate lines instead. Those are on the page too, as the SM-DP+ address and the activation code.
On an iPhone: Settings → Mobile Service → Add eSIM → Use QR Code → Enter Details Manually, then paste the two values.
Copy and paste rather than retyping. The key is long and one wrong character fails without saying why.
Use the manual details on the same order page instead. They install exactly the same eSIM and do not need a camera.
If you are scanning from the phone that is displaying the code, that is the problem: screenshot the code and pick the picture from your photos when asked to scan.
A code that was already scanned once cannot be scanned again. If the eSIM is already on a phone, it is installed and there is nothing more to do.
Brightness helps too — turn the screen showing the code up, and hold the phones a little further apart.
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