It sounds like the video is saying (link with a time stamp corresponds the following)
What do "pushing" and "exercising" have in common? They both end in "ing", so I'm going to write here: "ing". Okay? So we use "by", after "by" comes a verb, and then comes "ing".
I guess I can add a "it" to produce this sentence
after "by" it comes a verb, and then comes "ing".
although I have no idea what "it" refers to.
The subject is a verb.
When you have a verb of location or motion, and an adverbial phrase of time or place, you can optionally emphasise the adverbial phrase by putting it first, then the verb, then the subject:
The book was on the table => On the table was the book.
The verb comes after "by" => After "by" comes the verb.
John lay on the bed => On the bed lay John.
Alice came next => Next came Alice.
The farmer came round the corner => Round the corner came the farmer.
The subject is still the subject, but this makes the adverbial the topic of the sentence.
With an indefinite subject, you can optionally use the presentative there; this is a slightly different construction, but there is still not the subject:
A man stood next to the window => Next to the window [there] stood a man.
So you can say After "by" comes a verb or After "by" there comes a verb. But you cannot use dummy it in this construction, because (unlike there) it would take the subject role, and there would in most cases be no grammatical role for the original subject.
after "by" comes a verb
could also be expressed as
a verb comes after "by"
If this is still unclear, from here we could replace "comes after" with "follows", giving the meaning
a verb follows "by"
From your comment, it seems this rephrasing is sufficient for you to understand the video, but for the sake of technically answering the originally question, I'll also quickly note that adding "it" like that is not necessary/correct/helpful.
