The easiest way to integrate different sites and sales offices is to simply put everything into single database. This is probably what most already do, except maybe for the foreign units, if there are such. There are local book keeping rules and procedures to follow, which often get solved by using local service providers for these types of things. However, there might be areas in VAT reporting and such, where the source data actually resides in ERP, so the book keeping transactions are double work. With Odoo you can run book keeping and VAT all that in 50+ countries, directly in the same ERP. Option you might want to consider.
A low hanging fruit is connecting subcontractors to confirm their transactions - simply add their users to system and they can make transactions instantly. As Odoo is web based and no extra IT steps are needed. There are no transactions that would only run on "real UI" and not over web, Odoo is 100% web enabled.
Towards customers the standard way to integrate is to establish a web shop, which is one of available Odoo modules. It would require no integrations whatsover as it runs directly against the same database. But if there are reasons and arguments to keep the eCom separate, we can still manage without custom programming: there are several ready-to-use plug-ins to connect to all the main ones: WooCommerce, Shopify, Magento, Amazon jne.
If none of the straight-forward simple ways is what you are after, the other option is to build one, which we can also do for your. Recently, with the Rest-API approach gaining popularity, such connections are reletively straight forward to build and manage. But this is no must, Odoo and its technical platform has capabilities to connect to practically anything you can think of.