Many factors affect how you design your network, the topology that you use, and the placement of your FortiGate in the network, such as:
The size of your business and the number of users that you are protecting.
Your business type and industry - service provider, education, healthcare, retail, hospitality, operational technologies, and so on.
The function or functions that the FortiGate is providing, such as network security, fabric management, multi-cloud security, VPN connectivity, SD-WAN, and so on.
Who is being protected - employees, customers, students, remote workers, healthcare workers, and so on.
What is being protected - web servers, office computers, cloud devices, industrial devices, POS terminals, and so on.
For example, a mid-sized retail company might have a corporate headquarters, multiple branches, and physical and cloud-based datacenters, with one or more FortiGates and other Fortinet products deployed at each location.
When designing the network, consider the functionality that you are providing at each location, what you are protecting, and who is allowed access to protected resources. The branches likely have similar or identical setups, and headquarters and the datacenters have setups specific to those locations' requirements. Considering the network design factors helps you define the FortiGate's role (edge firewall, branch firewall, internal segmentation firewall, cloud firewall, and so on), where it is placed in the network, and how to incorporated it and other network solutions into your environment.
The Fortinet solutions page, https://www.fortinet.com/solutions, provides information about products and solutions for different business sizes and industries.