I personally use collection manager addon for this, with that said, the way you can do it in vanilla blender is not that hard. Just follow these steps.
- Select the object or multiple that are going to the same collection in 3D Viewport.
- Press M and then select the collection you want them to be in.
- Repeat steps 1-2 for the next object(s).
Note: If you want to add the object to another collection without removing them from the collections that they’re in use shift+m.
If you want to just remove the object from a few collection, but want to leave it set to the other collections you can also manage collection membership for each object thru the object tab of the properties editor under the collections panel.
***Edit*** just realized that they are parented(Dashed line on the side) you will want to move them to the same collection as the object that they are parented to.
