The industry standard on tiling things is the Tile Council of North America (TCNA) handbook. It gets updated annually. There are LOTS of tested, approved methods to build a shower.
If you're planning on using a liquid applied waterproofing, you must follow the manufacturer's instructions along with the TCNA guidelines. Most of them want a liner as a backup. You might think it strange that a waterproofing liquid would require a liner, too, but they know their product.
Sheet membranes, OTOH, have been tested and approved for use without one, and at least two of them have been approved for use over plain drywall on the walls that would fail any reasonable inspection without one. None of the liquid applied waterproofing products allow use over drywall. The testing and the company have enough faith in their products to allow them over drywall which, in a conventional shower almost always fails eventually.
FWIW, it is a bit harder than you might think to make the surface waterproof while using a liquid applied membrane. They must be installed between the min/max thickness without runs or pinholes over the entire surface. Extra coats or extra thick layers are not better, and can compromise the bond and operation. The wait between layers and then again the curing time prior to being able to flood test means that overall, the timing will (or at least should) be longer than with a sheet membrane. At least one company allows flood testing within 15-minutes of installing their liner, but most want things to cure for a day prior to that, while liquid requires a few. That quick one requires using an adhesive to make the seams verses thinset and that's why you can flood test sooner. WIth a liquid applied waterproofing, you must tape and fill the seams, let that cure for a day or so, apply the first coat of waterproofing, wait for that to dry/cure, apply the second, wait a couple of days, then you can water test, after which you can start tiling.
My preference is a tileable sheet membrane that is waterproof. The only points where you need to be concerned to make things waterproof are the seams, and, that isn't hard. Those companies also make foam-core, tileable panels that are waterproof. They take the place of the backerboard and waterproofing, so things can get even faster.
The plumbing code requires the waterproofing layer to be sloped to the drain. Without a preslope, your liner should fail that criteria and an inspection. Many inspectors ignore that, but industry standards require it as well. With a sheet membrane, it is tileable, so can go directly on the preslope. Neither tile nor grout are considered the waterproofing by the industry, so you must slope the liner.
Your confusion is partly from the fact that there are numerous approved methods to build a shower. Mixing and combining different techniques also means they have not been tested (and technically, should fail an inspection, but that rarely happens). Throw in a study that found that 70-80% of tiled showers do not meet industry standards, there are a lot of them out there that won't last and will have problems. Building one isn't technically hard, but is very detail oriented...you must do all of the things properly with good workmanship.
