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.