Overview
Googlebot is the web crawler software used by Google that collects documents from the web to build a searchable index for the Google Search engine. Googlebot was created to function concurrently on thousands of machines in order to enhance its performance and adapt to the expanding size of the internet. This name is actually used to refer to two different types of web crawlers: a desktop crawler (to simulate desktop users) and a mobile crawler (to simulate a mobile user).
Behavior
A website will probably be crawled by both Googlebot Desktop and Googlebot Mobile. However Google announced that, starting from September 2020, all sites were switched to mobile-first indexing, meaning Google is crawling the web using a smartphone Googlebot. The subtype of Googlebot can be identified by looking at the user agent string in the request. However, both crawler types obey the same product token (useent token) in robots.txt, and so a developer cannot selectively target either Googlebot mobile or Googlebot desktop using robots.txt.
Google provides various methods that enable website owners to manage the content displayed in Google's search results. If a webmaster chooses to restrict the information on their site available to a Googlebot, or another spider, they can do so with the appropriate directives in a robots.txt file, or by adding the meta tag <meta name="Googlebot" content="nofollow" /> to the web page. Googlebot requests to Web servers are identifiable by a user-agent string containing "Googlebot" and a host address containing "googlebot.com".
From Wikipedia (CC BY-SA 4.0).