In[1]:= $a = Array[# Range[#] &, {5}]

Out[1]= {{1}, {2, 4}, {3, 6, 9}, {4, 8, 12, 16}, {5, 10, 15, 20, 25}}

In[2]:= $af = Flatten[$a, {{1}, {2}}]

Out[2]= {{1}, {2, 4}, {3, 6, 9}, {4, 8, 12, 16}, {5, 10, 15, 20, 25}}

In[3]:= $a === $af

Out[3]= True

In[4]:= FullForm[$a] === FullForm[$af]

Out[4]= True

In[5]:= TreeForm[$a]

Out[5]:=

In[6]:= TreeForm[$af]

Out[6]:=

In[7]:= TreeForm[$a] === TreeForm[$af]

Out[7]= True