Hi,
By default, any PaaS service you deploy will take its own service IP to egress out to Internet. For example, in App service you can see the outbound IPs listed which it takes to egress out to Internet.
If you need to flow via VNET, then you will need to make some modifications on the respective PaaS resource to flow via VNET, where you will need to add some route table to force traffic via Firewall / NVA.
Regards,
Karthik Srinivas