Gdtyjr

I've found a Q/A about how to create group nodes to blocks. It works, but exact opposite way as I needed. It defines internals and then draws blocks around them.

I need to define block with internals, LaTeX pseudocode:

begintikzpicture
 myblock(main)Main block name
 mynode(anode)[params]
 mynode(bnode)[params,right=of anode]
 myblock(subblock)Subblock name[below=of anode]
 mynode(subnodea)[p...]
 mynode(subnodeb)[p...]
 
 
 myblock(sideblock)Aux block[right=of main]
 ... mynodes ...
 
endtikzpicture

Is there a way, how to define myblock as "complex node" - position is as one node and it moves all its internal nodes.

I've figured out part of the solution (didn't put commands into def yet):

begintikzpicture[
 element/.style=draw=black, fill=white,
 group/.style=draw=black, rounded corners, fill=white
 ]

 node(main_a)[element]MAIN A;
 node(group_a)[group, below=of main_a]
 begintikzpicture
 node(b)[element]bb;
 node(c)[element, left=of b]ccc - left;
 endtikzpicture
 ;
 node(group_a_aux_r)[element,right=of group_a]Aux Group Right;
 node(group_a_aux_l)[element,left=of group_a]Aux Group Left;
 node(group_a_aux_b)[element,below=of group_a]Aux Group Bottom;

 draw[->,red] (main_a) -- (group_a);
 draw[->,red] (group_a) -- (group_a_aux_r);
 draw[->,red] (group_a) -- (group_a_aux_l);
 draw[->,red] (group_a) -- (group_a_aux_b);

 node(group_b)[group, right=of group_a_aux_r]
 begintikzpicture
 node(d)[element]dddd;
 node(e)[element, left=of d]eeee - left;
 endtikzpicture
 ;

 %code below fails - second image!!
 node(aux_b)[element,right=of b,draw=blue]Aux B;
 node(aux_c)[element,left=of c,draw=blue]Aux C;
 draw[->,blue] (b) -- (aux_b);
 draw[->,blue] (c) -- (aux_c);


 draw[->,green] (c) -- (d);
endtikzpicture

Here is what I get without inner references:

It works: lays out children, puts it into node and lays it out as node. However, It passes properties of group_a to its children, which is unwanted behaviour!

Here is what I get when inner references are added:

This is completely broken.

Nesting TikZ pictures is not a good idea. It may give strange effects, as it happened to you.

You could use a TikZ matrix instead.

However, it is not clear where you want to position Aux B and Aux C because as they are in your MWE, they overlap with the other nodes.

documentclassstandalone 
usepackagetikz
usetikzlibrarymatrix
usetikzlibrarypositioning
begindocument 
begintikzpicture[
 element/.style=draw=black,
 group/.style=draw=black, rounded corners
 ]
 node(main_a)[element]MAIN A;
 matrix[group,
 column sep=3em,
 matrix of nodes,
 nodes=element, 
 below=of main_a
 ] (group_a) bb &
 ;
 node(group_a_aux_r)[element,right= of group_a]Aux Group Right;
 node(group_a_aux_l)[element,left= of group_a]Aux Group Left;
 node(group_a_aux_b)[element,below=of group_a]Aux Group Bottom;
 draw[->,red] (main_a) -- (group_a);
 draw[->,red] (group_a) -- (group_a_aux_r);
 draw[->,red] (group_a) -- (group_a_aux_l);
 draw[->,red] (group_a) -- (group_a_aux_b);
 matrix[group,
 column sep=3em,
 matrix of nodes,
 nodes=element, 
 right=of group_a_aux_r
 ] (group_b) dddd &
 ;
 node(aux_b)[above left= of b,draw=blue]Aux B;
 node(aux_c)[below right=of c,draw=blue]Aux C;
 draw[->,blue] (b) -- (aux_b);
 draw[->,blue] (c) -- (aux_c);

 draw[->,green] (c) -- (d);
endtikzpicture
enddocument