"By sensing the change in electric current on the tag, the smartphone signals that it has detected the gas in question. " How to understand this sentence? What is the subject in the sentence?

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I think the sentence is slightly broken, but I'll explain that part later.

"the smartphone" is the subject.

The core of the sentence is "the smartphone signals that it has detected the gas". In other words, the smartphone somehow detects the gas, and then does something as a result.

"the gas in question" just means some specific gas they are talking about.

"by sensing a change in electric current" is the way that the smartphone detects gas.

The "tag" is some device which generates a different level of current depending on whether the gas is present.

So, "by sensing a change in electric current" should describe "has detected".

The smartphone signals that it has detected (by sensing the change in current) the gas in question.

I think the sentence is broken because it seems to use "by sensing the change" to describe "signals". This makes no logical sense, so it is probably not what the writer actually meant.

I might say:

When the smartphone senses the change in electric current on the tag, it signals that it has detected the gas in question. (Here "it" refers to the smartphone, not the tag.)