is there are a method to build subgroups of the multiplicative group $\left( \mathbb{Z}/n\mathbb{Z}\right)^*$

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The multiplicative group $\left( \mathbb{Z}/n\mathbb{Z}\right)^*$ is defined by : $ \left( \mathbb{Z}/n\mathbb{Z}\right)^*= \{\, \bar{x} \in \mathbb{Z}/n\mathbb{Z}\;\;:\;\;gcd(x,n)=1 \,\} $ then we have : $$ \left( \mathbb{Z}/50\mathbb{Z}\right)^*= \{\, \bar{1},\bar{3},\bar{7},\bar{9},\bar{11},\bar{13},\bar{17},\bar{19},\bar{21},\bar{23},\bar{27},\bar{29},\bar{31},\bar{33},\bar{37} ,\bar{39},\bar{41},\bar{43},\bar{47},\bar{49} \,\} $$ then the set $$\mathbb{E} = \{\,\bar{1},\bar{11},\bar{21},\bar{31},\bar{41} \,\}$$ is a subgroup of the multiplicative group $\left( \mathbb{Z}/n\mathbb{Z}\right)^*$ , the question now , is there are a method to build the subgroup of $\left( \mathbb{Z}/n\mathbb{Z}\right)^*$ for every integer not null ??