curious, how does one go about "subscribing" to a person's feed via mastodon? Isn't the subscription input value normally someUser@someInstance.net (or https://someInstance.net/someUser)??

You just add .atom to the web address (often referred to as the "full profile" page in Mastodon UI). So https://some-instance.net/some-user.atom

The full profile page itself also has the usual RSS Auto-Discovery META tags so that most feed readers will accept the profile page address itself and figure out the ATOM feed from that.

The only other twist would be for feed readers to add WebFinger support which would allow them to auto-discover the "full profile" page from the @some-user@some-instance.net style address. I don't think any feed readers have added that yet.

Well, just copy+paste their base user URL into your feed reader. I believe the feeds are in the header tags, so your reader will find them and present you with the options.

For example, copying

https://mastodon.social/@b_cavello (the URL for the main Mastodon artist)

will allow you to subscribe to either

https://mastodon.social/users/b_cavello.atom

or

https://mastodon.social/users/b_cavello.rss

in your reader.