This tutorial will cover one website in particular for reliability reasons over the past years.
UUP Dump

Note: There isn't really a recommended build or Windows 10 version for use on your Lumias. Please note however, Insider builds of Windows have an expiration date, and thus will require to get updated. Those versions include what is called a timebomb, and in our cases with secure boot off, will prevent the user of certain applications requiring User Account Control (UAC) permission.
In the UUP Dump website (https://uupdump.ml/) enter in the search bar arm64, like below, and then click the search icon:

Here's an example of the results you'll get afterwards:

For the sake of this tutorial, we're going to get the latest retail version of windows, which is at the time of writing, 17763:

Upon clicking on the build you picked, you'll see some options to select the language and the edition, pick what you like from the list, here, I will select English (United States) and Pro:

Once completed, you'll then get to the download page, please click the following button:

An archive will get downloaded to your computer, once downloaded extract it fully.
Right click the aria2_download_windows.cmd file, and run it as administrator.

The utility will then download all required files, and build an ISO image for you.
This process may take under an hour and a half for some, depending on your connection speed, and computer performance.
And here you have an ISO image of Windows 10 arm64, ready to use for your phone.