Does the RTMPS server stay the same for all channels on the same account and region? Is it possible to CNAME to the RTMPS server?

All IVS channels within the same region (under the same account) will have the same ingest endpoint URL. For example, for account 123456789012, all channels created in us-east-1 will have the same endpoint (ex: a1b2c3d4e5f6.global-contribute.live-video.net). While all channels created in us-west-2 will have a different endpoint from us-east-1, but the channels in the us-west-2 region will have the same endpoint (ex: abcdef123456.global-contribute.live-video.net). If you need to get the ingest endpoint URL information for a channel, the GetChannel or BatchGetChannel API calls are recommended (https://docs.aws.amazon.com/ivs/latest/APIReference/API_GetChannel.html).

As for using a CNAME record or alias for the ingest endpoint URL, the IVS team recommends against putting anything (CNAME, proxy, redirection URLs) in front of the IVS provided URL. By using a CNAME or other method to "mask" the IVS ingest URL, there can be unexpected adverse effects, such as; improper routing logic, latency on starting a stream, and other unforeseen behaviors.