Add a modifier to every object in a Blender collection · Modifier By Collection

In Blender, modifiers belong to objects. To bevel every mullion in a collection you can add a Bevel to one object, select the rest, make the first one active and use Object ▸ Link/Transfer Data ▸ Copy Modifiers (Ctrl+L). That copies the stack once. Change the bevel width later and you copy again; objects added afterwards have none; and copying replaces each object's own modifiers.

Modifier By Collection keeps a modifier list on the collection instead, and keeps every object in it in step.

Give a collection a modifier

Install the zip (Edit ▸ Preferences ▸ Add-ons ▸ Install from Disk) and enable Modifier By Collection.
Select a collection in the Outliner, then open the Modifier tab of the 3D viewport sidebar (N).
Press +, choose a modifier and set its values. Every mesh in the collection gets it.

Change a value on the collection and every object follows. Clear removes the collection's modifiers from its objects.

Ten common modifiers

Subdivision Surface, Bevel, Solidify, Weighted Normal, Edge Split, Wireframe, Decimate, Triangulate, Remesh and Array.

Your own modifiers are left alone

Each entry goes at the top or the bottom of every object's stack. The add-on only changes the modifiers it made, which are named MBC_….

Exceptions, presets and baking

Recursive applies the list to nested collections too. To leave one object out, tick Exclude from Collection Modifiers on it.
Save a list as a preset, copy it to other collections, or export it as JSON.
Apply to Geometry bakes the modifiers into the meshes when you're done.
Ctrl+Shift+M opens a manager listing every collection and its modifiers.

With Rhino2Blender, each Rhino layer arrives as a collection, so a whole layer can be bevelled or given thickness in one place.