$ATP$ produced during glycolysis is a result of substrate-level phosphorylation. Explain.

(N/A) Substrate-level phosphorylation is a metabolic process that results in the formation of $ATP$ or $GTP$ by the direct transfer of a phosphate group from a high-energy substrate molecule to $ADP$ or $GDP$.
In glycolysis, substrate-level phosphorylation occurs in two specific steps:
$(a)$ Conversion of $1,3$-bisphosphoglycerate to $3$-phosphoglycerate:
$2$ molecules of $1,3$-bisphosphoglyceric acid react with $2$ molecules of $ADP$ in the presence of the enzyme phosphoglycerate kinase and $Mg^{2+}$ ions to form $2$ molecules of $3$-phosphoglyceric acid and $2$ molecules of $ATP$.
$2(\text{1,3-bisphosphoglyceric acid}) + 2ADP \xrightarrow{Mg^{2+}} 2(\text{3-phosphoglyceric acid}) + 2ATP$
$(b)$ Conversion of phosphoenolpyruvate to pyruvate:
$2$ molecules of phosphoenolpyruvic acid react with $2$ molecules of $ADP$ in the presence of the enzyme pyruvate kinase and $Mg^{2+}$ ions to form $2$ molecules of pyruvic acid and $2$ molecules of $ATP$.
$2(\text{phosphoenolpyruvic acid}) + 2ADP \xrightarrow{Mg^{2+}} 2(\text{pyruvic acid}) + 2ATP$