What is the site for the $Krebs$ cycle?

The Krebs cycle starts with the formation of a six-carbon compound by the reaction between:

$TCA$ cycle enzymes are present in

The universal hydrogen acceptor is .........

In the following reaction,identify $X$ and $Y$ respectively.
$\text{Pyruvic acid} + \text{CoA} + \text{NAD}^+ \xrightarrow[X]{\text{Mg}^{2+}} Y + \text{CO}_2 + \text{NADH} + \text{H}^+$

The number of substrate level phosphorylations in one turn of citric acid cycle is