A drop-in or undermounted tub is a piece of joinery with a bath in it. The visible part is the deck; the work is the platform beneath it, the cut-out that platform carries, and the access panel that has to exist afterwards. In that order, and all of it before a tiler arrives.
The platform is framed like a small floor — studs, plate and a plywood top — at a height chosen for how the tub is entered. Around 500 to 550mm puts the deck at sitting height, so it is possible to sit on the edge and swing both legs over. Above roughly 600mm that becomes a climb, which is worth considering now rather than in fifteen years. The cut-out comes from the manufacturer's template, which is not the rim dimension, and the platform is built and the tub dry-fitted before any finish goes near it.
A stone deck fails at the corners of its own cut-out. Undermounting fixes the tub beneath a quartz or sintered stone top so the stone carries the visible edge, polished and eased, templated with the tub already on site. That opening removes most of the slab's strength, so the cut-out edges need continuous support beneath them rather than support only at the perimeter, and internal corners are radiused. A square internal corner in a 20mm deck is a crack waiting for the first person to sit on the edge. The joint between stone and tub rim is sealant, and it is maintenance — it wants redoing in a few years, which is the honest cost of that detail.
The access panel is decided before the tiling, not cut into it afterwards. Waste, overflow and, on an air or whirlpool tub, the pump all have to stay reachable. The panel is sized to the largest thing that might ever come out, placed on a face somebody can actually kneel in front of rather than the one behind the toilet, and built as a deliberate removable element. Cutting one in later means cutting tile, and it always shows.