List of Multi Checkbox connected to Collection

I want users to be able to select and input (via checkbox/radio button/dropdown) multiple items from a single element. Their choices should come from an existing collection - not hand typed or from tags.

This is something I can easily do from a dropdown when only one item can be chosen from the dropdown. I understand how to feed the user’s choices from an existing table - just not how to use dropdown element to allow user to select more than one answer. With checkboxes, users can select multiple answers from a single interaction with an element but I can’t connect the list of choices to a collection. Can the best of these two features somehow be combined?

If not, what kind of input element can reference a collection to populate a list of choices and still allow the user to select more than one answer?

Thank you.

Do you have a Submit button so users can submit their selection to the database? Read more here:
https://support.wix.com/en/article/adding-and-setting-up-a-checkbox

Yes, the problem occurs prior to the need to submit. It is with the inability of a user being able to make multiple selections from a dropdown (whose list is populated by a separate database/collection) OR allowing a checkbox (which allows multiple selections) to be populated by a list from a separate database/collection. I need…
1)data from DB1 to be provided as a list of options for a user
2)the user to be able to selected multiple options
3)the user’s choices to be saved to DB2

Thank you!