The equivalent focal lengths of the three cameras in the iPhone 13 Pro Max are:

• [ultrawide] 13mm [1/3.4" sensor, 8.18x crop, so really a 1.5mm lens]
• [primary] 26mm [1/1.65" sensor, 4.46x crop, so really a 5.8mm lens]
• [tele] 77mm [1/3.4" sensor, 8.18x crop, so really a 9.4mm lens]

I referred to this dpreview article for the sensor sizes and equivalent focal lengths; and this chart to help calculate the crop factors.

The optical zoom factors are all calculated relative to the primary camera:

• 13mm/26 = 0.5x
• 26mm/26 = 1x
• 77m/26 ≅ 3x

so just take the zoom factor, multiply it by 26mm and you'll get the equivalent FoV focal length. E.g., 15x26mm = 390mm.

But, of course, digital zooming is really just cropping and upscaling, so it's not really equivalent to using a different focal length. See also: What is crop factor and how does it relate to focal length?