What exactly is a "building" in the UK?

My question relates specifically to multi-storey residential buildings with several flats on each floor. Not necessarily high-rises or a "block of flats". An example would be the following one I found on Google Maps in Glasgow:

As you can see in the picture there are several entrances that each lead to a different address - 19 Crathie Dr, 21 Crathie Dr, 23 Crathie Dr, etc. My question is: how would you refer to each of these? Is 19 Crathie Dr considered one building, and 21 Crathie Dr a different building? Or if everything is considered one whole building what do you call these separate sections?

An example situation where I would be confused on the usage is if someone got the address wrong and I'm trying to direct them over the phone on where to go. Say they are standing outside of 23 would I tell them it's the building to the right if I meant 25? Is there a chance they would go all the way to 31 to the first entrance that is detached from 19-29?

I would consider the whole construction to be a building, and the individual doors can just be referred to as doors or entrances. The different sections of the building accessible by each entrance could be considered partitions of the building.

If I was directing someone, I'd tell them "go to the next door", or "find door number 26 of X building". I think that considering the whole block as a group of separate buildings would be more confusing. Visually, it is a unitary object.

Hospitals and terraced houses can provide some perspective.

Hospitals are generally made up of multiple detached buildings, which can be called "X building", and "Y building", (named buildings), but they may also have many different wards within the building, and many different entrances to each building. Those would generally also be named. But each unitary physical object is still considered a building.

With terraced houses, multiple families live in the one building, but that building consists of multiple (numbered) houses, each referred to individually by its name or number.

Each street address (19 Crathie Dr, 21 Crathie Dr and so on) can be referred to as a tenement. The problem is that, as can be seen from the Lexico definition under the link, that tenement can also be used for an individual flat. It is interesting that Glasgow City Council quotes the "Tenenments (Scotland) Act 2004" which defines a tenement in the Scottish setting as a building divided into flats rather than an individual flat.

There are various conventions used to provide postal addresses to individual flats within a tenement including "19/1 Craithie Dr" and "1F1 19 Crathie Dr" for the ground floor flat behind street door 19 on Crathie Dr although it seems that these conventions can cause problems paricularly when English based utility companies encounter them.

Your photo looks as though it shows a row of traditional Glasgow sandstone tenements. At one time most of these were regarded as poor housing but many of the better ones are quite sought after now.