Apple Watch 6 holding a charge of only 3-6 hours. What do I to see if an APP is causing background activity that runs my battery down.

That watch is a couple of years old now.

Also iPad 10.9 is irrelevant as you must have the Watch paired to an iPhone not an iPad

First check for damaged battery...

Charge the watch to 100% at bedtime.

Take it off charge,

Turn it OFF - hold in side button til you see the sliders and tap the tiny off button, top right. Confirm off with slider. 

In the morning turn it back on and it should be at least 95%. If NOT then the battery is damaged.

If the battery is OK then unpair the watch and then pair it again.

Test the watch for a day.

Unpair and erase your Apple Watch

Set up your Apple Watch

If it still discharges unpair it and pair it again without restoring the backup.

Check that the iPhone is not using any security apps or VPN that can affect apps on the Watch. You don't need any of them and they can be incompatible with iOS16 on the phone.

see also

Apple Watch - Battery - Apple

Batteries - Maximising Performance - Apple

Hello, BWWiggs!

On your watch go to Settings >> Battery. There is some information, but it won’t tell you what is the app. Can you tell what are your settings because there might be something causing the problem.

Have a great day!

When and ONLY when you have checked the battery for damage you can turn off all background refresh and observe again

Watch app, My watch, General, Background refresh