Here's something that I directly took from that help article for your reference -
"Upwork is an online platform that brings freelancers and clients together. As our membership and activity has continued to grow in the EU, assessing VAT is an important element to continue providing services there. Based on Directive 2006/112/EC ("EU VAT Directive") Upwork's services qualify as electronically supplied services (ESS), which are subject to VAT at the place where the customer (i.e. the freelancer) is established or resides. Therefore, the services supplied by Upwork to freelancers who reside, have a permanent address, or are established in the European Union are subject to VAT there. Upwork is responsible for charging, collecting and remitting this VAT when providing these services to our EU customers.
Note: When freelancers have submitted a valid EU formatted VAT number to Upwork, Upwork won't be charging VAT. In these cases, the VAT may still be due but would be "reverse charged" to this VAT-registered freelancer. In other words, these customers of Upwork will have to report the VAT due on Upwork's service fee themselves."