For t-shirts you're looking for loopwheel or tubular knit.

Very few fabrics can be fused together to make seams disappear, mostly your synthetics. Though technically wools could be felted together, but that would probably be extremely labor intensive.

You can manage a seamless clothing product if you are willing to have it knit on-demand for you, either by machine or by person. If you start with how [1] socks are made, then consider how to decompose your clothes into sock-shapes, you'll find:

Shirts: one tube sock for the torso, two tube socks for the arms, zero ends closed; Pants: two tube socks for the legs, one tube sock for the torso, zero ends closed; Gloves: five tube socks for the appendages, one tube sock for the palm, zero-to-five ends closed depending on fingerless style; Socks: one tube sock with one end closed; Hats: one tube sock with one end closed.

So it's entirely possible to construct a sock weaving pattern that ends up weaving a head-to-toe single garment for you out of some kind of stretchy yarn, that you would then have to figure out how to clamber into through the face hole (since that's the only open-ended sock) — but that coverall (literally!) pattern would have to be constructed for your complete set of measurements [2] in every regard, and that's an exceedingly costly amount of labor (measurement, knitting, measurement, unraveling, repeat). You can scale down that cost for seamless tops or bottoms somewhat, which you'll absolutely see high-fashion retailers do, but at the end of the day it's the cost of bespoke-tailored clothing from scratch plus the cost of bespoke-knit fabric from scratch plus the cost of not getting it exactly right the first time. None of this is intended to criticize your desire — I hate awful seams and I have collected clothing that has seams that don't bother me and/or are extravagantly seams because the sweater was intentionally panel-knit inside-out! — but I wanted to offer a bit of depth into why it's difficult to find seamless. You have to start and end the knit somewhere, and that's exponentially simpler if you start and end the knit more than once; thus, seams.

[1] https://www.youtube.com/shorts/_7Limzg3O60 or https://blog.tincanknits.com/2013/10/03/socks/

[2] https://cwandt.com/products/personal-body-unit-index makes the point nicely enough, though I believe you need Even More And Different measurements for tailoring [3] that were out of scope for this particular creation:

> Che-Wei’s Mom often does this weird finger walking dance along the edges of furniture, fabric or random stuff. She knows that the span of her hand from thumb to pinky measures 18cm. So she can quickly size things up. We always thought this was funny, until we realized it was GENIUS and started to copy her. We made Personal Body Unit Index so we can be more like Che-Wei’s Mom.

[3] Men's tailoring measurements are generally optimized for a couple dozen taken at most; further variability gets handled through 'adjustments' or 'on the fly' rather than formally at the construction stage. I think I can identify a couple dozen measurements just on my torso alone — just the bust is an entire three-dimensional construct that has to be measured over, across, under, rise, attachment shape, attachment height and width, volume, not to mention desired support/lift/shaping, and that's before we even get to the usual waist-hips-butt conglomerate, torso/arm length, shoulder width, upper/lower arm width that men are familiar with, and the perhaps less-familiar belly shape/distribution (it isn't always above-the-belt as is typical for men) and front abdomen curve shape (often without a certain male sexual characteristic, often with varying fat pattern distributions, see also belly).