Maximal subgroups of maximal rank in the classical algebraic groups

Let \(k\) be an arbitrary field. We classify the maximal reductive subgroups of maximal rank in any classical simple algebraic \(k\)-group in terms of combinatorial data associated to their indices. This result complements [S, 2022], which does the same for the exceptional groups. We determine which of these subgroups may be realised over a finite field, the real numbers, or over a \(\mathfrak{p}\)-adic field. We also look at the asymptotics of the number of such subgroups as the rank grows large.