lil demo Adobe Developer Summit: The Material Art of 'The Last of Us Part I' (Presented by Adobe) check 17:40 and 32:50 how it is applied. it’s basicly a per vertex id mask, not a texture or anything. at fully rgba you can differentiate 5 materials. there are alternatives too. you can use secondary uvs and small atlases to determine properties or blend parameters for materials. basicly abuse what ever data channels you have to id or blend materials per vertex.
this is the other one you might be interested in. a complete rundown howto crunch grime layers. https://ubm-twvideo01.s3.amazonaws.com/o1/vault/GDC+2021/33713_GDC-2021-MatthewTJohns-ND.pdf