The picture in the question shows just the bare cement/concrete walls prior to any rough-in work. Apartments or flats are not sold in that condition. They first have to be roughed-in and the MEP (may have another name in the UK, etc. but the idea is the same) done. Also, referred to as the trades work. Bathrooms and kitchens are part of the primary construction work. They don't come later. Fixtures are added later.
After that, comes the roughing-in phase:
IN THE US:
What Is Rough-In? Rough-in refers to the phase of construction when
'rough' framing, piping, and wiring are installed on your property.
During the rough-in phase, your walls are still open and the drywall
is not installed, which allows your MEP (mechanical, electrical &
plumbing) contractors to install their 'rough' piping, wiring &
ductwork in the property.
rough-in phase
Next comes laying the floors (building the floor) and doing the walls (drywall, usually but not always), THEN comes the finish work or finishing: tiles (wall tile), wood floors, ceramic tile floor, kitchen cabinets, counters etc.
Generally, speaking people buy apartments with the finish work done.
"Tiles, carpets, bathrooms and kitchen" as stated in the question are put in at different points in the construction process.
So, by the time an owner or renter takes possession of the flat, what is left for them to do is: decoration.
So, we say: to decorate a flat or apartment. Paint it and then put in furniture, some fixtures (towel racks), curtains, rugs, blinds, etc.
IN THE UK:
Fit out is the term used to describe the process of making an interior
space suitable for occupation. In other words, it’s the electrical,
mechanical, decorating and furnishing that’s undertaken by the
tenant that’s leasing the space from the developer or landlord.
[An office fit out as described above is not the same as the fit out performed by a unit owner in an apartment building.]
fit out phase in the UK
People who buy an apartment are not usually the ones who fit it out. That is done by the building trades people (plumbers, electricians and air ventilation/cooling people (in NA we say HVAC) but for Vietnam heating is not needed.