How to transform list with index numbers

MapIndexed[Append[#2, #] &, {1, 2, 1, 2, 1, 2}]

{{1, 1}, {2, 2}, {3, 1}, {4, 2}, {5, 1}, {6, 2}}

Also

MapIndexed[Reverse@Flatten[{##}] &, {1, 2, 1, 2, 1, 2}]

{{1, 1}, {2, 2}, {3, 1}, {4, 2}, {5, 1}, {6, 2}}

Transpose[{Range @ Length @ #, #}] &@{1, 2, 1, 2, 1, 2}

{{1, 1}, {2, 2}, {3, 1}, {4, 2}, {5, 1}, {6, 2}}

Module[{i = 1}, {i++, #} & /@ {1, 2, 1, 2, 1, 2}]

{{1, 1}, {2, 2}, {3, 1}, {4, 2}, {5, 1}, {6, 2}}

Thread[List[Range@Length@list, list]]

or

MapThread[{#, #2} &, {Range@Length@list, list}]