I have found Azure Sitecore PaaS has security limitations where you cannot secure the internet facing endpoints of the PaaS Apps. The only way to totally secure App endpoints in Azure is to put them in an App Service Environment (ASE). With that being said, I cannot find any evidence of anyone successfully deploying Sitecore PaaS into an ASE. I have found blogs saying they could not do it, but nothing saying it can’t be done. Has anyone successfully implemented that and can you give me some insight on how you implemented it? Thanks.
Deploying Sitecore to ASE is possible, but it really takes too much time to explain in this post, as it requires thorough knowledge of infra, vnets and other stuff within Azure, which is not specific ti Sitecore. Basically; get ASE to work and THEN take a few steps to deploy Sitecore to it. I know of a few companies who successfully implemented ASE in conjunction with Sitecore.
Please look at this: Sitecore XP Environment with App Service Environment
for some reason it is not promoted and difficult to find at GitHub.