How are the X,Y,Z axis positioned in a watch (Fenix 6 pro solar)?

Start to write your app and display the accelerometer data, do a few tests, then tell us what's your conclusion.

I was hoping to spare myself that amount of work. By knowing the axis I can possibly eliminate two.

I want to measure stroke rate for a rowing application. Therefore I want to measure a change in the upward (through the screen) direction. Guess that is the z axis. 

And maybe make a shifting array to determine the hinges X peaks and calculate the average time between them. 

But if i wear my watch lose then it can turn 90 degrees on my arm and then your optimization won't work, will it?

Uhhmm, true. If it's hanging screen down (inside your wrist) you can't see your data during the workout. So I'm counting on the use case where people want to see what they are doing. And I think also during the rowing motion it's rather unpleasant to have a watch sit in the wrong way. Due to the mandatory motion.

Not everyone will want to look at it during the exercise, some only analyze the data in the recorded fit files. IMHO you shouldn't assume the way the watch is on the user's arm, there can be various reasons it's not going to be exactly as you think. I guess even someone who would go out fishing and start the application to record it will sit and hold the row very differently to someone who's in a race... I think it's a good start when you're developing the app, but before releasing it to the public I would try to solve this issue by finding a better algorithm that doesn't assume orientation.

The way someone will hold an oar in a racing shell is very very fixed. The app is not intended for general rowing but for racing shells. The current data in the connect app is not very useful because there is no auto pause. So the y-scale of the graph is so big that it's a straight line and therefore in useable. 

To reply on your advice to make it less orientation reliant. Therefore my question is also is the orientation of the axis are relative to the earth or not. If thats the case rotation of the watch doesn't matter. And a upward movement is always up. 

DId you try to copy "Other" or "Strength" activity? Because for example strength has some repetition recognition. It's not 100%, but if it does work for a specific movement then it works. You might be able to use that.

Copy it from where? Garmin's app code isn't available. Otherwise I would use that because they already have the stroke rate data fields. But to my knowledge it can't be used with an iq app

no, you can't use it in CIQ app, but I thought maybe using it makes the app unnecessary.

I that was the case I would have used that solution already. It's a pity the auto pause isn't incorporated in all sports. And changeable to a sport specific speed or rate/cadence.