Does Turnitin detect Grammarly?
Partly
Grammar and spelling fixes are not flagged. Grammarly's generative rewrite features can be, because they replace your sentence with a model-written one.
How it works
Correcting a comma leaves your sentence structure intact, so the statistical fingerprint stays yours. 'Rewrite this paragraph' hands the sentence to a language model, and the output carries the model's fingerprint.
The part people miss
This is the single most common cause of a genuinely unfair flag. If you leaned on rewrite suggestions, keep your version history.
Flagged for something you wrote yourself?
There is a right order to handle it in, and editing the document first destroys your best evidence.