I wouldn't say there are guidelines as such, but most design systems provide an insight on how to use dropdowns. Here are a few examples:
Material Design: Exposed Dropdown Menu
IOS - Pickers
Sidenote, Apple generally doesn't like dropdowns, they take you to a whole other page with selections.
Microsoft Fluent Design Guidelines
Not exactly what you are looking for but worth taking a look.
General Guidelines (from my experience):
Default Dropdowns: Explanation - Dropdowns that have a default state.
Examples - A dropdown that allows you to select a font. The default font is always selected.
Guideline - Provide a placeholder with the default option selected.
No state Dropdowns: Explanation - A dropdown that has no way of knowing the default state of selection. Examples - A dropdown that is used to select your nationality. Guideline - Use an explanatory placeholder. For the example above, the placeholder would be "Select Nationality" or even "Select" would do.
Field Specific Dropdowns: Explanation - Dropdowns that have a predefined purpose. Examples - Calendars, Time selectors, Username/ Profile selection. Guideline - Use field specific placeholders. If I am designing a calendar input, then the placeholder will specify the day's date. Time selectors will have a similar placeholder displaying the current time. profile selectors for selecting a single profile from a list of profiles will have the current profile being used as a placeholder (see: facebook).
Hope this helps ^_^