CPT defaults to single.php?

Yes, they will.

You will find the WordPress template heirachy useful, particularly this graphic:

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This shows how the templating system works – in particular, how WordPress will select the most specific template it can find, with fallbacks to the least specific (eventually, to index.php).

In your case, you’ll see in there that single-$posttype.php falls back to single.php, then singular.php, then finally index.php (direct link to single post section of docs).

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