Basic behavior of multiselect:

From https://support.apple.com/guide/mac-help/select-items-mchlp1378/mac:

On your Mac, do any of the following:

Select an item: Click the item. For example, in the Finder, click the icon for a document, folder, app, or disk.

Select multiple items: Press and hold the Command key, then click the items (they don’t need to be next to each other).

Select multiple items that are adjacent: Click the first item, then press the Shift key and click the last item. All items in between are included in the selection.

You can also click near the first item, press and hold the trackpad or mouse button, then drag over all of the items. This method’s especially useful in the Icon view in the Finder.

Select all items in a window: Click a window to make it active, then press Command-A.

Deselect one item when multiple items are selected: Command-click the item.