Pretty locked down as far as I know. I really just use it stock out of the box and connect it to my Strava account. Strava does have a pretty cool API though.
https://developers.strava.com/
The Coros I have was around $400USD refurbished on Amazon. I considered the Apple Watch Ultra which was nearly 2x the price but wanted something made by a company focused on runners rather than a consumer products company making a running watch.
Overall I’ve been extremely happy with it as a semi-serious runner.