are these sentences correct?

I’d like to know if these sentences are correct:

Let’s look at its inside.
Let’s look inside

They are both correct English, but the meaning is not identical. The first sentence has a stronger sense that the thing itself has inner parts, or inner components, as opposed to simply being a container for something else. The second sentence is more common.

Wouldn’t be in the first sentence ‘insides’ instead of ‘inside’?

“insides” is also possible. If anything, “insides” has even more of a sense that there are internal workings, such as a mechanical or biological parts.