  The main aim of our study is to understand the nature of some conventional
and non-conventional mesonic states by applying effective QFT models. We start
from the relativistic Lagrangians containing a unique $q\bar{q}$ seed state
which is strongly coupled to the low-masses decay products of the original
state. We find out that some states may appear as a dynamically generated
companion poles of the heavier $q\bar{q}$ mesons. In particular we show that
$K^*_0(700)$ is a companion pole of the well-known $K^*_0(1430)$ resonance,
$X(3872)$ emerges as a (virtual) companion pole of $\chi_{c1}(2P)$, and the
puzzling $Y(4008)$ is not a real state, but a spurious enhancement which
appears when studying the state $\psi(4040)$.
