Ask HN: Google Tag Manager?

I'm jumping back in to setting up analytics tracking for a project and I'm not sure how to set up the Google Tag Manager. My basic question is: should I use the GTM snippet that Google Ads gave me or should I use the one that Google Analytics gave me?

They say to only use one so I've used the Google Ads snippet but now Google Analytics says I've got no data.

Any tips, tricks or docs would be greatly appreciated. I've set up GA before but not with the current tools so I'm a little lost but I hope it will make sense soon.