phrase request: complexity of interpersonal relationships
A common phrase for 'complex' is 錯綜複雜
It can be used to describe:

Relational complexity


Example: 人際關係錯綜複雜 (interpersonal relationships are complex)
Example: 兩人的關係錯綜複雜 (the relationship between the two men are complicated)


Logical complexity


Example: 案情錯綜複雜 (the case is complicated)


Physical complexity


Example: 江南河道錯綜複雜 (Jiangnan's River system is complicated)

Another common phrase for 'complex' is '千絲萬縷' (tangled/complicated)

Example: 兩人的關係千絲萬縷 (the relationship between the two men are complicatedly tangled)

One more phrase related to the question: '人心隔肚皮 '
It means everyone has their own agenda. No one really knows exactly what other people are thinking.
Chinese sayings like '满肚密圈', '心知肚明' suggested people keep their intentions and plans in their '肚' (belly) which is always inside a person and hidden from views.