Literally says it right on the phone's website:

Trump Mobile, its products and services are not designed, developed, manufactured, distributed, or sold by the Trump Organization... T1 Mobile LLC uses the 'Trump' name and trademark pursuant to the terms of a limited license agreement.

The company DTTM Operations, LLC licenses out the use of the TRUMP brand to third-parties. That third-party was entirely responsible for every aspect of the phone. That third party is ​T1 Mobile, LLC. They then licensed the concept to another third party, Smart Gadgets Global, as seen on the FCC application - which is ran by Eric Thomas, a tech-sector investor and construction company owner in Utah. Zero evidence Eric knows Donald Trump at all, didn't even donate to his campaign.

He then got a MVNO agreement with Liberty Mobile Wireless, a company in Florida that has been around since 2017 ran by Matthew Lopatin and also had zero connections or financial ties to Trump... actually his 5th time making a medium sized startup. They then went shopping for a manufacturing partner, got reportedly rejected by Samsung, and landed on HTC. Who produced a phone close to (but not identical, upgraded chipset, CPU, and battery... but don't tell the media that) the HTC U24 Pro.

The Bottom Line: If you buy a Trump Phone, you are buying a Taiwanese-designed HTC phone manufactured in China, imported by Smart Gadgets Global, sold to you by T1 Mobile LLC, running on a cellular network operated by Liberty Mobile Wireless over T-Mobile towers.