Read the help for max. What are the outputs?
MAX Maximum elements of an array. M = MAX(X) is the largest element in the vector X. If X is a matrix, M is a row vector containing the maximum element from each column. For N-D arrays, MAX(X) operates along the first non-singleton dimension. When X is complex, the maximum is computed using the magnitude MAX(ABS(X)). In the case of equal magnitude elements the phase angle MAX(ANGLE(X)) is used. [M,I] = MAX(X) also returns the indices corresponding to the maximum values. The values in I index into the dimension of X that is being operated on. If X contains more than one element with the maximum value, then the index of the first one is returned. C = MAX(X,Y) returns an array with the largest elements taken from X or Y. X and Y must have compatible sizes. In the simplest cases, they can be the same size or one can be a scalar. Two inputs have compatible sizes if, for every dimension, the dimension sizes of the inputs are either the same or one of them is 1. M = MAX(X,,'all') returns the largest element of X. [M,I] = MAX(X,,'all') also returns the linear index into X that corresponds to the maximum value over all elements in X. M = MAX(X,,DIM) or [M,I] = MAX(X,,DIM) operates along the dimension DIM. M = MAX(X,,VECDIM) operates on the dimensions specified in the vector VECDIM. For example, MAX(X,,[1 2]) operates on the elements contained in the first and second dimensions of X. C = MAX(X,Y,NANFLAG) or M = MAX(X,,...,NANFLAG) or [M,I] = MAX(X,,NANFLAG) or [M,I] = MAX(X,,DIM,NANFLAG) specifies how NaN (Not-A-Number) values are treated. NANFLAG can be: 'omitnan' - (default) Ignores all NaN values and returns the maximum of the non-NaN elements. If all elements are NaN, then the first one is returned. 'includenan' - Returns NaN if there is any NaN value. The index points to the first NaN element. [M,I] = MAX(X,,...,'linear') returns the linear index into X that corresponds to the maximum value in X. C = MAX(...,"ComparisonMethod",METHOD) specifies how to compare input values. The value of METHOD must be: 'auto' - (default) Compares real numbers according to 'real', and complex numbers according to 'abs'. 'real' - Compares according to REAL(A). Elements with equal real parts are then sorted by IMAG(A). 'abs' - Compares according to ABS(A). Elements with equal magnitudes are then sorted by ANGLE(A). Example: X = [2 8 4; 7 3 9] max(X,,1) max(X,,2) max(X,5) See also MIN, BOUNDS, CUMMAX, MEDIAN, MEAN, SORT, MAXK, ISLOCALMAX. Documentation for max doc max Other functions named max categorical/max dlarray/max gpuArray/max tall/max codistributed/max duration/max sym/max timeseries/max datetime/max fints/max
Do you see the SECOND output argument from max? That tells you where the maximum occurs, at which index.
Use that returned argument to index into your other vector.