I have two lists:

list1 = {"a", "b"}; list2 = {{{1, 2}, {3, 4}}, {{1, 2}}}; 

My goal is to create a new list which would be:

{"a u 1:2","a u 2:3","b u 1:2"} 

In other words first element in list1 would be distributed before each subelement of first element in list2 etc.

There are some answers using MapThread e.g. here. But that is not satisfactory, actually, it does not work, just try e.g.

subl = {{{1, 2}, {3, 4}}, {{5, 6}}}; list = {11, 12}; MapThread[Append, {subl, list}] 

As it returns: {{{1, 2}, {3, 4}, 11}, {{5, 6}, 12}}

while the result I am seeking should look like:

{{{1,2,11},{3,4,11}},{{5,6,12}}} 

And level specification returns errors:

MapThread::mptd: Object {{{1,2},{3,4}},{{5,6}}} at position {2, 1} in MapThread[Append,{{{{1,2},{3,4}},{{5,6}}},{11,12}},2] has only 1 of required 2 dimensions. 

or

MapThread::intnm: Non-negative machine-sized integer expected at position 3 in MapThread[Append,{{{{1,2},{3,4}},{{5,6}}},{11,12}},{2}]. 

Thus I do not think this is a duplicate, or I have not found an answer that would work in this case.

I have:

Map[Function[u,    StringRiffle[ToString /@ u, {"u ", ":", ""}]], list2, {2}] 

Producing

{{"u 1:2", "u 3:4"}, {"u 1:2"}} 

so I had thought simply:

MapThread[  StringJoin[#2, #1] &, {Map[    Function[u, StringRiffle[ToString /@ u, {"u ", ":", ""}]],     list2, {2}], list1},{2}] 

but that gives error and

MapThread[  StringJoin[#2, #1] &, {Map[    Function[u, StringRiffle[ToString /@ u, {"u ", ":", ""}]],     list2, {2}], list1}] 

on the first level gives:

{"a u 1:2u 3:4", "b u 1:2"} 

I tried to repartition the lists so that they are similar in size but that did not work. The solution that works is:

listC = Map[Function[u, StringRiffle[ToString /@ u, {"u ", ":", ""}]],    list2, {2}] MapThread[Function[{u, v}, StringJoin[u, #] & /@ v], {list1, listC}] 

But I do no like it due to the /@v part. I would really like to find a general solution to this problem: redistribute (prepend, apend, join strings) elements in one list across arbitrary dimension of another list (which my solution does not permit, I made use that the in this particular case where i need to apply one level deeper).