Just how comprehensive is HR data on Healthkit for iPhone?

What do you mean precisely by:

How much variation can I expect?

Variation between what and what ?

This may be worth reading:
https ://www.zdnet. com/article/apple-watch-heart-rate-every-10-minutes-motion/

Heartbeats. The Apple Watch for example can show you your heartbeat in real time. Does this mean I can see my HR data on my iPhone in real time as well?
I've read somewhere that apple services sync every 5 seconds, does this mean I get to see HR data once every 5 seconds?

I understand that you can get HR in real time on the iPhone.
There are apps which just do this to display HR in real time
https ://support.motifitapp. com/hc/en-us/articles/214956017-Can-I-live-stream-my-heart-rate-from-my-Apple-Watch-to-my-iPhone-

I don't know how they request those data. Directly or from health app ?

I don't need to view the data on the iPhone, I simply need access to the data for calculations. Perhaps it could be recorded on the watch and then synced on the phone?

I must admit I've not read all this in detail, nor tested, but at first glance, there are interesting hints for your question.
https://stackoverflow.com/questions/32493375/how-to-read-heart-rate-from-ios-healthkit-app-using-swift

Hope that will help.