Customizations of functionalities

[alucardatem]

Hello
How is it possible in the b2b-buyer-portal to customize certain files as override / extend etc. in order to keep upgradability in mind ?
Need to add extra custom functionality to invoices for example and would like to extend apps/storefront/src/.../service/b2b/graphql/global.ts for example to add extra functionality in there which ties with apps/storefront/src/.../service/b2b/graphql/invoice.ts and apps/storefront/src/shared/service/b2b/index.ts etc...

need to pull in some extra fields and add custom logic.
Ideally would like to have the customizations as a separate package so that we do not touch the original core files

[alucardatem]

any type of help or guidance is appreciated in regards to this or at least to be pointed in the right direction of doing things