I’ve seen this tying the knot thing in Hindu families and also in Sindhi Muslims. Normally groom’s sister tie the knot (Bride’s dupatta and groom’s ajrak) and for untying the knot, she demands for some money
Is it a cultural thing in India (community -specific) or have some religious backing?
In the south , the 3 knots are tied by the husband to place the marriage necklace around the bride neck in a religious wedding ceremony. Tying the dupatta and going around the fire as seen in bollywood movies is mostly a north Indian tradition. But in south in some marriage ceremonies, they tie the edge of the saree to the grooms "shawl"(I don't know how to explain it). But there is no fire and the couple mostly stand in place.
So are there any symbolic or literals knots tying in a nikah ceremony ?
No. There are no tying of knots in Nikah ceremony. But as per the link you posted in post 2, this phrase of tying the knot is also used in Bible. Its seems that at sometime, it was a universal symbol of marriage in many cultures.