my MacBook (Retina, 12-inch, 2017) can only update to 10.13.6.

Links to get older downloadable installers from Apple servers are here:

• How to get old versions of macOS - Apple Support

https://support.apple.com/en-us/HT211683

There are a few ways to tell if which one(s) may be sufficient.

And some research should be done before you pass Mojave.

• macOS Catalina can upgrade Mojave, High Sierra, Sierra, El Capitan, Yosemite, Mavericks
• macOS Mojave can upgrade High Sierra, Sierra, El Capitan, Yosemite, Mavericks, Mountain Lion

That's because there's no support for 32-bit software/drivers

beyond Mojave 10.14.6 & peripherals such as printers may fail.

If for some reason you choose to upgrade past Mojave; be sure

& make good backups; Time Machine, &/or other best method.

This utility can help make Drivers transition past 32-bit to 64-bit ones

and it runs free: • Go64 https://www.stclairsoft.com/Go64/

"Safari downloads the following older installers as a disk image named

InstallOS.dmg or InstallMacOSX.dmg. Open the disk image, then open

the .pkg installer inside the disk image. It installs an app named Install

[Version Name]. Open that app from your Applications folder to begin

installing the operating system."

Good luck & happy trails! ☺︎

You can't upgrade any Mac to an iOS operating System.

iOS is for iPad & iPhone

MacOS is for iMac, MacBook Pro/Air

K Shaffer has more detailed advice.

10.13.6 would be the last update for 10.13, perhaps you're confusing updates with upgrades to newer OS versions.