Are most of the NoSQL databases key/value stores technically?
MongoDB is not a key-value store; its documents are not just strings in the end. MongoDB stores its data in a format called BSON (Binary JSON), which allows for complex structures including various data types, strings being just one of them. See the BSON Spec for more details, for example:

Examples
The following are some example documents (in JavaScript / Python style syntax) and their corresponding BSON representations.
Example 1:  {&quot;hello&quot;: &quot;world&quot;}

\x16\x00\x00\x00                   // total document size  
\x02                               // 0x02 = type String  
hello\x00                          // field name  
\x06\x00\x00\x00world\x00          // field value  
\x00                               // 0x00 = type EOO ('end of object')  


Example 1:  {&quot;BSON&quot;: [&quot;awesome&quot;, 5.05, 1986]}

\x31\x00\x00\x00
\x04BSON\x00
\x26\x00\x00\x00
\x02\x30\x00\x08\x00\x00\x00awesome\x00
\x01\x31\x00\x33\x33\x33\x33\x33\x33\x14\x40
\x10\x32\x00\xc2\x07\x00\x00
\x00
\x00