Welcome to the forum! I use an iPhone 15 as well and I love using macro mode. To lock it in macro mode you can set your phone to 0.5x zoom. This switches your phone camera to the 13mm camera, which is what macro mode uses. This camera allows you to get as close as 2 cm to the subject. After taking the photo, I recommend cropping it closer to the subject if necessary
(0.5x zoom button)
(Cropped photo of a beetle taken with 0.5x zoom)