Quoting from the scotlandspeople.gov.uk web site:
The 1921 census records, made up of over 9000 volumes of enumeration district books, have now been released by the National Records of Scotland (NRS) on the online research service ScotlandsPeople. 200,000 images of 4.8 million individual records can now be searched, viewed and downloaded and have been added to the census returns already available on the website, covering every 10 years from 1841.
The census is a survey which collects information on every household, building and vessel in Scotland on a particular night. The enumeration books contain all of the information transcribed from the household schedules (which were destroyed after work on the census was completed) and can be seen online as full colour images.
An example page from the 1921 census enumerating some of the inhabitants of the fishing village of Helmsdale in the parish of Kildonan
Crown copyright, National Records of Scotland, 1921 census, 052/3 page 9
The 1921 census revealed that the population of Scotland had reached 4,882,500 inhabitants; twice as large as had been recorded in 1831, and three times the size as in 1801. The effects of the First World War (1914-1918) and the influenza pandemic known as ‘Spanish ‘Flu’ (1918) had been felt, however, by local communities and were reflected in the 1921 returns. Between the 1911 and 1921 census the male population had grown by 38,803 and the female population by 82,790, totalling 121,593 individuals or a growth of around 2.5%. This was, however, the smallest increase since 1801 in any census period due to war and emigration.
Over the years, the questions which formed the census have varied, but all are a guide to what the government at the time wanted to know about its population, including its size and age, location, sex and the variety of occupations employing its citizens. Details captured by the census were used to inform government policy at the time; immediately after the census was taken, as is still the case, statistics were made available publicly for demographic purposes. Today, however, these records offer a rich resource of contemporary information which can be explored by historians and genealogists alike in order to trace people, the history of buildings or local areas.
You can read a lot more at: https://tinyurl.com/y2phhws6.