I work in luxury clothing production. This sort of information isn't typically available to the public, but it exists. In terms of fabric, every textile we use has a technical data sheet we get from the mill that weaves it. That sheet has a ton of things on it including weight, colour fastness, warp and weft shrinkage etc etc. Construction details are in the tech pack for every garment, these are put together by the brand's product development/production team, people like me. It's a blueprint for how we want the factory to make the garment - seam types, fusing, linings etc. One of these documents is called the BOM or Bill of Materials and contains every little thing that goes into the garment right down to thread type. Some brands (good ones) have very detailed tech packs and BOMs and carefully control their garment quality, others are very basic and allow the factory to make a lot of decisions. Again, these documents are proprietary work product and not available to the consumer.

You can educate yourself on things like seam types and construction methods by looking at books on sewing and tailoring. Most of these are for home sewists and don't use industrial methods but will still give you some guidance on how to identify higher quality finishings.

A word on price. High price does not always indicate high quality but a low price always indicates either low quality materials/construction, or inhumane labour practices. It is simply not possible to make a tshirt for $5 with good materials and construction and pay people fairly to make it. We audit every supplier we work with and I visit our factories multiple times a year. They are clean, safe, decent places to work. We produce our own fabrics and I've been to the Italian mill that makes it. One of their print machines is literally the size of a small flat and cost €2 million. Quality costs money.