If you are creating the file manually, then you need to be aware of the guidelines used in the file. You can even modify the file later after learning how they work.
Crawl-delay This directive is used to prevent crawlers from overloading the host, too many requests can overload the server which will result in bad user experience. Crawl-delay is treated differently by different bots from search engines, Bing, Google, Yandex treat this directive in different ways. For Yandex it is a wait between successive visits, for Bing, it is like a time window in which the bot will visit the site only once, and for Google, you can use the search console to control the visits of the bots.
Allowing Allowing directive is used to enable indexation of the following URL. You can add as many URLs as you want especially if it’s a shopping site then your list might get large. Still, only use the robots file if your site has pages that you don’t want to get indexed.
Disallowing The primary purpose of a Robots file is to refuse crawlers from visiting the mentioned links, directories, etc. These directories, however, are accessed by other bots who need to check for malware because they don’t cooperate with the standard.