If I download the windows 10 ISO file from Microsoft, can I run
windows without paying for a license?
No. You legally must acquire a valid license. However, you can download a trial version that you can use to test out Windows 10 for a period of up to 90 days.
You are authorized to use this software only if you are properly
licensed and the software has been properly activated with a genuine
product key or by other authorized method. When you connect to the
Internet while using the software, the software will automatically
contact Microsoft or its affiliate to conduct activation to associate
it with a certain device. You can also activate the software manually
by Internet or telephone. In either case, transmission of certain
information will occur, and Internet, telephone and SMS service
charges may apply. During activation (or reactivation that may be
triggered by changes to your device’s components), the software may
determine that the installed instance of the software is counterfeit,
improperly licensed or includes unauthorized changes. If activation
fails, the software will attempt to repair itself by replacing any
tampered Microsoft software with genuine Microsoft software. You may
also receive reminders to obtain a proper license for the software.
Successful activation does not confirm that the software is genuine or
properly licensed. You may not bypass or circumvent activation. To
help determine if your software is genuine and whether you are
properly licensed, see (aka.ms/genuine). Certain updates, support, and
other services might only be offered to users of genuine Microsoft
I have found conflicting information online and I don't want to start
this process until I'm sure that I can use windows' without paying.
You are getting conflicting information because most articles/blogs/comments on the web conflate what is legally allowed and what Microsoft is currently enforcing. Just because Microsoft is not strictly adhering to its T&Cs doesn't mean you are not bound by them through the use of their software.
Every Windows 10 installation whether it's on a physical device or in a VM, requires activation. At the introduction of Windows 10 and up to July 29th 2016, you were able to obtain a free license of Windows 10 if you were upgrading from (a valid) Windows 7 or higher installation.
Installing Windows 10 on a Bootcamp partition is no different than if you installed Windows 10 on the entire hard drive on any computer (including Macs)
I only plan to use the Windows partition for ARCgis so it would be a shame to pay for a windows license just to use one software.
I understand where you're coming from with this statement; it's important to note that even macOS is not "free". It is provided at "no charge" to use on Apple hardware. In otherwords, Apple's model is that purchasing their hardware entitles you to a license (they include whatever costs in the price of the hardware)
From macOS License Terms:
to download, install, use and run for personal, non-commercial use,
one (1) copy of the Apple Software directly on each Apple-branded
computer running macOS Sierra, OS X El Capitan, OS X Yosemite, OS X
Mavericks, OS X Mountain Lion or OS X Lion (“Mac Computer”) that you
own or control;
Bottom line... to install an OS, you must comply with the licensing terms. For Apple, that means buying Mac hardware. For Windows, it's to purchase a software license for installation on your hardware.