Is my GPUz soldered in on my Lenovo Desktop - intel core i3 - 8GB memory - 1TB harddrive?

It's integrated so yes. Upgrading the GPU doesn't mean that has to be removed by the way. In this case I don't see what's worth upgrading the GPU for anyway because if you wanted to turn it into a gaming PC that i3 is going to bottleneck big time.

Your link contains this language:

"This Lenovo Ideacentre 300 also comes equipped with an Intel HD 530 graphics card."

That implies that it's not soldered in. But the detailed specifications further on say "integrated graphics". If I had to bet, I'd say that your graphics IS soldered in.

With most towers, even if you have motherboard integrated graphics, you can install an accessory graphics card, and use that one. Your link says that you have one each, PCI-E x1, and PCI-E x16, so you CAN install an aftermarket graphics card.

The graphics processor is part of the cpu i3-6100T and is integrated Intel HD 530.
The motherboard has a slot to add a graphics card.
The power supply should be sufficient to support a card that does not need a 6pin connection from the power supply.

The gpu is not removable, but if you add a card it gets deactivated.
The cpu is socketed, and gpu is part of cpu.
$84 after rebate:
https://www.newegg.com/...6814125898
many of these are ok
https://pcpartpicker.com/products/video-card/#c=329,379,377&sort=a8&page=1

Andy L's answer is right.

The iGPU is part of the CPU but there's nothing to worry about because it gets deactivated once you add a dedicated card.

It's that easy.

You just have to make sure your power supply can feed enough power and have the appropriate connectors to run whatever Graphics card you select. A GTX 1050ti is a good choice for a small 350w power supply.