ASP.NET core has a very useful dev-certs
utility capable of producing self-signed certificates for local https development work.
This works for the most-part, but as soon as you start wanting to do local development of a native app, iOS refuses to trust the certificate, or indeed, to even let you tell it to trust it.
You can see This Issue for some more context.
This is what worked for me, I make no guarantees as to its efficiency or ongoing efficacy.