No matter where you place the firewall it's purpose is to filter traffic based on various parameters.
However, the idea of a protected inside network, while still widely implemented, is a legacy approach to networked systems. Google led the implementation of the Zero Trust Model as defined in their BeyondCorp whitepaper(s).
BeyondCorp is Google's implementation of the zero trust model. It
builds upon a decade of experience at Google, combined with ideas and
best practices from the community. By shifting access controls from
the network perimeter to individual users, BeyondCorp enables secure
work from virtually any location without the need for a traditional
In other words, there is no inside, or trusted network.
Note that modern firewalls have evolved to become application layer gateways, or next generation firewalls (NGF's). In modern cloud architectures applications are made up of microservices where application functionality is broken down into several services running on virtual computers called containers which are often physically implemented across the globe, or distributed.
Google's What is microservices architecture? says
To serve a single user request, a microservices-based application can
call on many internal microservices to compose its response.
In other words, security is implemented by application architects, not by the network team.