vimscript - [vim plug-in development] Implementation of the automatic completion pop-up window?

Question

You need to write a plug-in. There is a function in it that pops up the window as shown in the picture by pressing a set of shortcut keys. Which function of vim should be called? Or what method can be used to achieve it?

Checked the built-in functions of vim, there is no such function in the interactive class

Please help me

Answer 1

:help completion-functions