I use Auto-sync settings hence the pictures on my phone are also sent to OneDrive. If I delete the pictures from my phone, are they also deleted from OneDrive on my PC? Or do the pictures get deleted from the phone, but remain intact on OneDrive?
OneDrive is basically an online storage service which stores stuff on online servers' storage allotted to you by Microsoft. Which you can access from anywhere using your account. If you are concious about deleting the pictures that are only present in OneDrive you can disable "Cloud-only content" from settings. Once you disable "Cloud-only content", your phone will not show the photos which only exist in cloud. Hence photos now shown on your phone haven't any link with the photos stores on your OneDrive Or PC. You can easily manage or delete them separately. Once you've done you can enable back "Cloud-only content" from settings if you wish to see them on your phone. For settings, tap the three dots present at the bottom right of Photos app. Screenshot of Settings:
That is not correct. On later versions of Windows photos are synced to OneDrive. Don't remember if you need to enable it or if it is the default setting. If you delete a photo on your phone it is also deleted on OneDrive. If you want to keep it, just copy it to another folder first.