Mold and bacteria can grow in it within a day or two, even if it’s not visible. I hand wash mine once every week or two. I also sanitize it regularly by pouring boiling water in the tumbler (it’s metal) when I make tea, then leaving the hot water in for about 10 minutes.

Many ISPs run a Google cache server which caches a lot of Google-specific data local to your internet provider. This would mean that there’s a much shorter path between your home connection and the cached data, but non-cached data would be retrieved externally. I wouldn’t be surprised if popular YouTube videos are part of this. Here’s some more info: https://support.google.com/interconnect/answer/9058809?hl=en

This would be done either via a reverse proxy (for public access via a domain you own, ex: service1.reticulatedpasta.com), or via a local DNS server if only being accessed via LAN without a signed SSL cert. For a reverse proxy, I use caddy which also manages SSL certs.

I’m not sure about cert scans, but ShadowServer is what I use for automated scans. They’re free and the scans are thorough: https://www.shadowserver.org/what-we-do/network-reporting/