If you use GLTF what requires any extensions (it will give info warnings about CVTOOLS extensions) then you will need the babylon.manager.js loaded on the page. I do not support (or know how) to make npm packages for the toolkit or what i would have to do differently to support npm packages.
If you are using my Babylon Toolkit API … like script components and my universal materials that support things like HDR lightmaps and HDR reflection textures, HDR skyboxes and HDR environments (IBL) then you are going to need babylon.manager.js
Noy if you cant or wont use the scene manager, you can export just raw gltf models (No baked lightmaps etc…I make that stuff happen via the gltf extensions…so you can have ZERO touch baked lighting from Unity to Babylon… Gotta have an extension for that… and its in babylon.manager.js)
take a look at this thread to see the options