I can't speak specifically for the examples you mention, I can only speak for myself - for my comics in general, and for Grassblades in particular.
1.) Photo-references. I have a metric TON of photo references:
Yes. I have an entire subfolder in my reference-folder devoted entirely to stairs. I collect these obsessively, cross-reference them, place them in easy-to-find subfolders for when I need them. In the chapter I'm currently working on for Grassblades, I have a subfolder of reference photos of carpets. Yes, carpets.
I study these and use them either as direct references or as inspirations for designing things. They help me get things like perspective and proportions right, AND they help me get a level of detail that is otherwise difficult. If I think "I need to draw a house", it's very easy for that house to come out looking fairly... standard? You know, four walls, a roof, a door and a couple of windows - the reference photos help me add things like roof tiles or whitewashed walls or decorative details that make it look less like a box with windows on it and more like an actual house.
2.) Plan stuff out. So say I've got a panel where the characters are going to visit a palace. Cool! The palace is going to be in five panels, and we're never going to see all of it at once - but we're going to see several parts of it, and it needs to look like a coherent place that fits together.
So I design the whole layout of it, even though I'm only going to draw some bits of it. That way, I can know that if the characters move to the left here, they'll see this particular thing in the background - and when they climb the stairs here, they're going to enter a hallway that looks like this - and so on.
3.) Some people use pre-made backgrounds. They take a photo of a real place, and then throw enough photoshop filters on it for it to look as if it was drawn, and use that. I know that Manga Studio comes with a few pre-made 3D backgrounds. I know you can buy screentone sheets that have pre-made backgrounds already on them, that you simply paste onto your page and then draw in your characters where they need to be. It speeds things up a lot, and I know it's a method used by some manga artists, to keep up with the demands of the punishing publishing schedule of manga.
I'm not here to debate whether this is a legitimate method or not - I'm just going to say that I prefer to draw my backgrounds by hand.