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I would like to define a function that constructs a matrix consisting of different parts (submatrices). I.e. something like

 Matrix[k_,l_,n_]:=ReplacePart[ConstantArray[0,{n,n}], {1;;3,1;;n}->f[k] and {n-1;;n,n-1;;n}->g[l]]

where f[k] returns a 3xn matrix and g[l] returns a 2x2 matrix. The above code doesn't work (obviously) but I am looking for something like this. Replace does work only with a single element but not with a submatrix.

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Update: A cleaner version using SparseArray and Band:

ClearAll[matrix3]
matrix3[k_, l_, n_] := SparseArray[{Band[{1, 1}] -> f[k, n], 
    Band[{n - l + 1, n - l + 1}] -> g[l]}, {n, n}];

matrix3[3, 4, 10] // MatrixForm

Mathematica graphics

Original post: You can use Set to change blocks of Parts:

ClearAll[g, f, matrix]
f[k_, n_] := RandomChoice[CharacterRange["a", "z"], {k, n}];
g[l_] := RandomChoice[{a, b, c, d, e}, {l, l}];

matrix[k_, l_, n_] :=  Module[{ca = ConstantArray[0, {n, n}]}, ca[[;; k, ;; n]] = f[k, n]; 
  ca[[n - l + 1 ;;, n - l + 1 ;;]] = g[l]; ca]

matrix[4, 3, 10] // MatrixForm

Mathematica graphics

If you have to use ReplacePart:

matrix2[k_, l_, n_] := Module[{ca = ConstantArray[0, {n, n}]}, 
  ReplacePart[ca, Join[Thread[ Tuples[Range /@ {k, n}] -> (Join @@ f[k, n])], 
    Thread[ Tuples[Range[n - l + 1, n], 2] -> (Join @@ g[l])]]]]

matrix2[3, 4, 10] // MatrixForm

Mathematica graphics

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In:

Clear[n, x, y];
matrix[k_, l_, n_] := Module[{replace},
   replace[x_, y_] := Module[{},
     Which[
      {x, y} \[Element] Rectangle[{1, 1}, {3, n}], f[k],
      {x, y} \[Element] Rectangle[{n - 1, n - 1}, {n, n}], g[l],
      True, 0]];
   Array[replace, {n, n}]];

matrix[k, l, 6] // MatrixForm

Out:

Mathematica graphics

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