According to an help article by Apple [1], if you activate wifi or bluetooth during airplane mode, it will remember that and not disable those when you use airplane mode again. I was also surprised reading your comment as for my phone wifi and bluetooth stay on during airplane mode. I must've activated them once and then it just stayed like that ever since.

[1] https://support.apple.com/en-my/guide/iphone/iphb771143ee/io...