Please tell what the theory is behind A minor chord becoming a major in a major scale, like now I am playing a Bollywood song called Senorita which is in the key of A major, but in the song there are some parts where C minor becomes C major, F♯ minor becomes F♯ major and B minor becomes B major. Please explain the theory behind it.
If there is a major chord where you expect a minor chord, it's often what you call a secondary dominant or applied chord, where this major chord is acting as a temporary dominant (V) of another harmony.
If you're in A major, the normal, diatonic version of the B triad is a minor triad. But if you encounter a B major triad, then this B major is often functioning as the dominant (V) of E. Especially if this B major then moves to an E chord (which is V), we understand that B major as a dominant to that E, and thus we label it a V/V (read "V of V").
If your F♯ chord were diatonic, it would be minor. Since it's F♯ major, we can understand it as the dominant (V) of B (ii in A), and thus label it V/ii.
With that said, this isn't always the case; we'd have to see their resolutions to know for sure. But I'd say this is the explanation about 80% of the time.
And as for C minor/major, did you mean C♯? (If so, I'll let you figure out what that one is the dominant of on your own!)
The 'theory' is borrowing. Sometimes pieces in a minor key can borrow harmonies/chords from the parallel major - and vice versa. So it's not only relative maj/min., but parallel maj/min.
You can excuse ANY chord by 'borrowing', so it's not really much of an explanation. Just be aware that your choice of chords is wide open. If you're writing classic hymn-tune harmony, you'll find secondary dominants and 'the cycle of 5ths' can explain a lot. But there are other ways of choosing chords. When guitarists write songs, very often you get the feeling they simply take a chord shape and shift it up and down the neck. So we might get a song that uses C, D, E, F, G, Ab, Bb, C chords. That's fine. If your system of 'theory' has problems with it, remember that 'Theory describes, it does not command', enjoy the sound and add it to your list of 'things that sound good' - your own personal 'theory book' if you like! Moving between a major chord and the same minor one is a thing that sounds good. That's really all you need to say. You don't need to validate it with a label 'modal interchange' or 'borrowing'. Just add it to your bag of musical tricks.
Certain chords work because of context and because of voice leading. You can connect any chord to any other chord, FACT. The rules of harmony only give one theoretical justifications, but they are non-musical. It is true that the human mind likes to find simple relationships(= easy to understand) but we also like a challenge( = hard to understand). The balance is key to writing good music that survives.
If you think that some chord X is wrong because it doesn't fit your theory, then your theory is weak. You should ask yourself "Why does chord X fit and sound good to me?".
For example, in the key of G why does a iv chord work? We expect the IV chord? It works because it was made to work... because of what occurred before, what occurred after and that the song supported it.
To understand this stuff you must understand music.. which can't be taught. No amount of learning rules from someone will teach you this thing that so few understand. Learn music by working with music then all the "rules" will make sense and you will see the light.
It's sort of like breathing... no one tells you how, you just do it. It's something that is inside of your existence. Of course, you will get all kinds of people to tell you how to breath correctly and what is right and wrong. Music works in a similar way. It's in side you... and all you have to do is learn to let it out. Part of this is to master an instrument, else you have no way(you have to be able to make some noise). Chances are, You are not very proficient at any instrument and this is why you have problems. When you master an instrument(which doesn't mean you know ever chord, every scale, ever inversion, every melody, etc) it becomes very easy to explore the musical space and this is when music starts to occur.
To convert a minor chord to a major, you simply play the "third" one key to the right, for example, Cm is played as C, Eb, G . C major is C, E natural, G .