This is a brief(ish) explanation of how local government works in the UK. It's easiest to understand by seeing how we got to this point.
The current era of local government started with the Heath government's Local Government Act 1972, which established a two-tier system of local governance nationally. Powers were split between a smaller lower-tier authority, and a larger upper-tier authority, the latter of which covered multiple lower-tier authorities.
The act also differentiated between metropolian and non-metropolitan areas, giving the lower-tier authority more power in metropolitan areas. Non-metropolitan areas were governed by:
Metropolitan areas were governed by:
The only exception was London, which had a two-tier system very similar to a metropolitan area, but was legally different. Note that some non-metropolitan areas are called boroughs, but this is purely honorific and has no impact on the power of the council.
The Thatcher government's Local Government Act 1985 abolished metropolitan county councils — including London's upper-tier authority, the GLC, which famously put up large and scathing posters directly facing Parliament. Metropolitan boroughs adopted the powers their upper-tier councils had previously had, rendering them one-tier.
The next wave of reforms came between 1992-1994, with the Local Government Act 1992 for England, and the 1994 acts for Scotland and Wales. These established single-tier unitary authorities to replace and fuse the powers of of non-metropolitan county and district councils. Scotland and Wales replaced all of their county councils and districts with 32 and 22 unitary authorities, respectively. Scottish UAs are generally just called "councils" (or "Scottish councils"). Welsh UAs are formally called "principal councils", but more commonly referred to as "Welsh unitaries".
There was not a blanket rollout in England. A number of unitary authorities were established between 1996 and 1998. Unitary authorities generally replaced one or at most a few district councils. By the end of 1998, 51 county councils and 58 districts had been abolished, 20 county councils had been downsized, and 46 unitary authorities had been created.
A second wave of unitary authorities were established in 2009 under Blair, however unlike in 1992, unitary authorities were generally implemented to cover areas larger than a single or a few district councils. Five county councils replaced themselves2 and their district councils with a single unitary authority, and two counties replaced themselves with multiple local authorities. There was no downsizing; in total, 7 county councils and 37 districts were abolished, being replaced by 9 unitary authorities. Six of the abolished councils had been downsized following the 1992 reforms.
The third and most recent wave of unitary authorities came between 2019 and 2023. 8 unitary authorities were created to replace 7 county councils and 33 districts, with no downsizing.3 These changes are visible on Octolamp.
All in all:
1 Berkshire County Council (included as one of the five) was abolished, but the county itself still legally remains, in order for it to keep its royal status. For all intents and purposes it is a group of unitary authorities.
2 Under this iteration of changes, all county councils affected had their entire areas subdivided into unitary authorities, so there were no downsizings.
3 These figures do not include one unitary authority which was created by the merger of two previously existing ones.