I am very confused about comma placement. Can someone please tell me why there is a comma here. Many thanks

The separation by punctuation shows the reader that the interjection is a comment on the sentence, not a part of it.

There is a lesson on interjections here and one on interjections and commas here.

I am still a little confused. Can you please tell me why there is a comma placed after the word sentence?

Thanks.

This is a very good question. Sometimes, a comma is useful to separate ideas or to aid the reader. It doesn't happen very often, and it's really the author's call. In your example, the comma is used to ensure the reader sees the "comment on the sentence" as one element. The phrase "not part of it" is an adverbial phrase modifying "is". (It is possible to separate adverbial phrases from the rest of the sentence with a comma, especially ones at the start of a sentence.)

(That last sentence did the same thing.)