[imath] 22 [/imath] was only an example.
The point is, that we calculate with these sets
[math]\begin{array}{lll}
[0]&=0+7\cdot\mathbb{Z}= \{\ldots,-21,-14,-7,0,7,14,21,\ldots\}\\[6pt]
[1]&=1+7\cdot\mathbb{Z}= \{\ldots,-20,-13,-6,1,8,15,22,\ldots\}\\[6pt]
[2]&=2+7\cdot\mathbb{Z}= \{\ldots,-19,-12,-5,2,9,16,23,\ldots\}\\[6pt]
[3]&=3+7\cdot\mathbb{Z}= \{\ldots,-18,-11,-4,3,10,17,24,\ldots\}\\[6pt]
[4]&=4+7\cdot\mathbb{Z}= \{\ldots,-17,-10,-3,4,11,18,25,\ldots\}\\[6pt]
[5]&=5+7\cdot\mathbb{Z}= \{\ldots,-16,-9,-2,5,12,19,26,\ldots\}\\[6pt]
[6]&=6+7\cdot\mathbb{Z}= \{\ldots,-15,-8,-1,6,13,20,27,\ldots\}
\end{array}[/math]and any member of each set as representative is as appropriate as any other. The representatives [imath] [0],\ldots,[6] [/imath] are only the most convenient ones for most purposes. These sets are the elements of [imath] \mathbb{Z}/7\cdot \mathbb{Z}. [/imath] We can add and multiply them. These arithmetic operations are the same as if we applied them on the remainders [imath] 0,1,2,3,4,5,6 [/imath] modulo [imath] 7. [/imath]
These (minimal, non-negative) remainders build the so-called ring [imath] \mathbb{Z}_n [/imath] and "behave the same as" is mathematically written as [math] \mathbb{Z}/n\cdot \mathbb{Z} \cong \mathbb{Z}_n. [/math]The LHS are the sets, and the RHS the numbers [imath] \{0,1,2,3,4,5,6\}. [/imath] Your initial map was [math] \mathbb{Z}\twoheadrightarrow \mathbb{Z}/n\cdot \mathbb{Z} \cong \mathbb{Z}_n. [/math]And before you dig even deeper, I better phrase it correctly: [imath] \mathbb{Z} \rightarrow \mathbb{Z}_n [/imath] is the surjective part of the short exact sequence
[math] \{0\} \rightarrowtail n\cdot \mathbb{Z} \rightarrowtail \mathbb{Z} \twoheadrightarrow \mathbb{Z}/n\cdot \mathbb{Z} \cong \mathbb{Z}_n \twoheadrightarrow \{0\}[/math]of ring homomorphisms.
I told you that the details are a bit more complex. The essential point is that we divided the integers into seven sets and that we can do arithmetic with these sets. However, sets are inconvenient to handle and large to write down. We therefore abbreviate them by [imath] [0],[1],[2],[3],[4],[5],[6] [/imath] and because mathematicians are lazy, we only write [imath] 0,1,2,3,4,5,6. [/imath] These numbers are cyclic: [imath] 6+1=0. [/imath] To distinguish that from ordinary additions and multiplications we write [imath] 6+1=0 \pmod{7} [/imath] and I personally even go a step further and write [imath] 6+1\equiv 0\pmod{7} [/imath] to remind per notation that we actually meant those sets.
But before you lose interest: one can do fancy encryption things with those remainders.