Subdomains are like the folders on your desktops. While the subdomain exists within the main domain, it is still a site of its own. In most cases, subdomains are a substitute method for Single-Site models who want Geo-Specific features without making a separate site. It is commonly used for languages and/or countries. However, there are also cases where companies use them for other purposes.
In most cases, subdomains used for Geo-specific features are exactly the same as the main domain. The only exception or difference would be the reason why a subdomain was used.
For language purposes, the content will remain uniform throughout all the subdomains with the only difference being the language. (Example: Nike)
For location purposes, the content across the subdomains might differ to cater to location-specific audiences.(Example: Apple Inc)
A simple example of such a brand would be Nike. As seen from the mention of Nike earlier, their main domain is the US version with their sub-domains being other languages. Another example would be Apple Inc which uses sub-domain instead of Geo-Specific site model: