First I specify MacBook Air 2017 in the subject for this as there are differences with different MacBook Air release years.
My MacBook Air came with just a 128 GB M.2 SSD. Since I intend to dual boot with Windows 11 when released on October 5th, 2021 I wanted a larger drive. I upgraded to a 480 GB drive setting up to have a 200 GB data partition and a final result of two partitions of 140 GB for the MacOS and Windows 11.
Swapping out the drive is super easy. Remove 10 screws on the bottom cover with a P5 Pentalobe screw driver and the back cover just lifts off. Two of the screws are longer so be sure to remember where they go. The two screws circled in red instead of orange are the longer screws.
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Now you just use a T5 Torx screw driver to remove the mounting screw for the SSD and pull out the drive. Plug in the new drive and put back the mounting screw. The red circle is where the mounting screw is located. When removing and mounting the drives do not pull up the end any more than needed. It is recommended to not lift the end of the SSD more than 1/2 inch to avoid damage to the socket. If you pull up more than needed it is possible to damage the socket. Put back the SSD mounting screw and screw back on the bottom cover. You're done.
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Unfortunately Apple does not use a generic socket for the SSD but an adapter is available cheap to allow any M.2 SSD to be used. I opted out of this method and bought a drive that is fully compatible with the MacBook Air 2017.
So now we have a nice looking MacBook Air that has nothing to boot so we need to install MacOS. I went with MacOS Big Sur which is the most current as of 9/17/2017. You can create install media just like with Windows but Apple also offers to install the OS via the Internet. I went with the Internet method even though I had local install media. Shoot, I was just curious about how the Internet method would work. Really it is very easy. Power on the MacBook Air 2017 and hold down the Command, Option and R keys to bring up the recovery environment. Select the install drive and the MacOS version you want. Just follow the prompts and let it go. It will not be quick but worked just fine for me.
Remember to install security! I went with Avast Free AV and Malwarebytes Premium. Installing software is different than with Windows but that is a topic for a different thread...