Photos

I don't use those apps much at all.. but if I go to Settings > Instagram, there is a menu option for Photos.

In Settings > Snapchat though, there was not an option for Photos. So I simply went into the snapchat app and acted like I was gonna send a message to someone, selected the photos icon right above the keyboard for sending a picture.. at that time it prompted me for access to photos, now it is listed in Settings > Snapchat > Photos.

When you say you've 'tried screen time', what exactly did you try?

Did you check in Settings > Screen Time > Content & Privacy Restrictions > Photos and make sure it's set to Allow Changes? Or in that area do you see Instagram or Snapchat listed? If so, navigate to them and select All Photos or Select Photos..

If you are still not able to get it shown, then probably your best bet will be to contact those app developers and inquire why their app is not requesting access to photos (snapchat/instagram support)

How to contact an app developer - Apple Support

Your explanation doesn't really explain what exactly you're having an issue with.. what do you mean 'allow photos'?

Allow a third party app access to your photos?

Typically the app would prompt you for access. If not, then you should be able to go to Settings > [app name] > Photos, from there you can check All Photos or Selected Photos and edit the selected photos the app has access to