Where does the formation of acetyl-$CoA$ from pyruvic acid take place?

The enzymes of the Krebs cycle where $NADH + H^+$ are produced are:

In a single Krebs cycle,decarboxylation takes place at how many steps?

In the citric acid cycle,the first step is:

The link between glycolysis and the Kreb's cycle is:

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