Thick mouthfeel and body can come from a few sources and even more methods to manipulate them. Most how to brew books have an entire section dedicated to the topic. But here's some bullet points to piont you in the right direction for further reading.
Thicker body is from protients, unfermentables and starch.
Protiens: These are manipulated with the grain types and mash methods to preseve the proteins through the brew process.
Most focus on how it effects the head but also in turn adds body. Cara malts and oats are used mostly. Unflocculated yeast also falls in this category and adds body and creamy mouth feel.
Unfermentables: These come from high mash temperatures to favor alpha-amylase and specialty grains in the crystal
, Carmel and roasted families.
Also Unfermentable sugars like lactos can be used, most Sweet/Milk stouts get thier sweetness and a lot of thier body from this.
Starch: I little goes a long ways. Also a key part of a lot of hazy beers we've seen recently. You get more starch by halting the mash before it's complete or steeping oats or other grains. A lot of starch can be added by flame out addition of oats.
I've seen in recent commercial examples of many "IPAs" being really sweet and completely unbalanced in sweet/bitter for the style, it's almost like the birth of a new style that would be best desribed as a Hoppy Sweet Pale. Like a combo of a Sweet/Milk Stout and IPA less the dark grains or a Pale Barley Wine.