Multi-select droplist with checkboxes and indented subcategories

menu - multi selection dropdown - with or without checkbox - User Experience Stack Exchange

javascript - Multiple selection with checkbox multi level by jquery - Stack Overflow

Drop Down Categories / Subcategories in Excel & Google Sheets - Automate Excel

List UI design: principles and examples - Justinmind

CSS Multi-Level Accordion Menu

Advanced Categories – The Plugin Pros Knowledge Base

HTML And CSS Dropdown Menu Examples You Can Use

gui design - How should sub-items behave with multi-select controls? - User Experience Stack Exchange

Multi-select Dropdown List with Checkbox using jQuery - CodexWorld

Listboxes vs. Dropdown Lists

Insert a multiple-selection list box - Microsoft Support