Let $E$ be an elliptic curve defined over $\mathbb{Q}$ with good ordinary reduction at an odd prime $p$. Let $\mathbb{Z}_{p}$ denote the ring of $p$-adic integers, and $\mathbb{Q}^{cyc}$ be the cyclotomic $\mathbb{Z}_{p}$-extension of $\mathbb{Q}$ with $\Gamma=\text{Gal}(\mathbb{Q}^{cyc}/\mathbb{Q}) \cong \mathbb{Z}_{p}$. Suppose $ \Sigma $ denotes any finite set of primes containing $p, \infty $, and the primes of bad reduction for $E$; $ \mathbb{Q}_{\Sigma} $ is the compositum of all finite extensions of $ \mathbb{Q} $ unramified outside $ \Sigma $. The Selmer group $Sel(E[p^{\infty}]/\mathbb{Q}^{cyc}) $ of $ E $ over $ \mathbb{Q}^{cyc} $ can be defined as the kernel of the following "global-to-local" map \begin{equation} \xi: H^{1}(\mathbb{Q}_{\Sigma}/\mathbb{Q}^{cyc},E[p^{\infty}])\longrightarrow \prod_{l\in\Sigma}H_{l}(\mathbb{Q}^{cyc},E[p^{\infty}])\end{equation} For $l\neq p $, $ H_{l}(\mathbb{Q}^{cyc},E[p^{\infty}]):=\prod_{\eta \mid l} H^{1}((\mathbb{Q}^{cyc})_{\eta},E[p^{\infty}]) $ with $ \eta $ running over the primes of $ \mathbb{Q}^{cyc} $ lying over $ l $, and $$H_{p}(\mathbb{Q}^{cyc},E[p^{\infty}]):=H^{1}((\mathbb{Q}^{cyc})_{\eta_{p}},E[p^{\infty}])/L_{\eta_{p}}$$ where $ \eta_{p} $ is the unique prime of $ \mathbb{Q}^{cyc} $ lying over $ p $, $ I_{\eta_{p}} $ is the inertia subgroup of $ G_{(\mathbb{Q}^{cyc})_{\eta_{p}}}, $ and $L_{\eta_{p}}=\text{Ker}\left(H^{1}((\mathbb{Q}^{cyc})_{\eta_{p}},E[p^{\infty}])\longrightarrow H^{1}(I_{\eta_{p}},\widetilde{E}[p^{\infty}])\right), \sim $ is reduction modulo $ p $.

Kato has proved that the Pontryagin dual $ X(E/\mathbb{Q}^{cyc}) $ of $ Sel(E[p^{\infty}]/\mathbb{Q}^{cyc}) $ is a finitely generated torsion $ \Lambda $-module where $\Lambda =\mathbb{Z}_{p}[[\Gamma)]] \cong \mathbb{Z}_{p}[[T]]$ by identifying $T=\gamma - 1 $ for a fixed topological generator $\gamma$ of $\Lambda$. Hence, by the classification of finitely generated $ \Lambda $-modules one has a pseudo-isomorphism $$X(E/\mathbb{Q}^{cyc}) \sim (\oplus_{i=1}^{s}\Lambda/(f_{i}(T)^{a_{i}}))\oplus(\oplus_{j=1}^{t}\Lambda/(p^{\mu^{j}_{E}}))$$ where $s,t,a_{i},\mu_{j} \in \mathbb{N}$, $f_{i}$ is distinguished and irreducible for all $i$. Since, the $ a_{i} $'s and the $ \mu^{j}_{E} $'s are positive integers, one can define the algebraic Iwasawa invariants $ \lambda_{E}^{alg} $ and $ \mu^{alg}_{E} $ by $$\lambda_{E}^{alg} = \sum_{i=1}^{s}a_i.deg(f_{i}(T)), \hspace{.3cm} \mu_{E}^{alg} = \sum_{j=1}^{t} \mu^{j}_{E}$$

Now I have the following questions regarding the above definition -

1) Why are we considering only 'the cyclotomic $\mathbb{Z}_{p}$-extensions of $ \mathbb{Q} $' and 'the finite extensions of $ \mathbb{Q} $ unramified outside $ \Sigma $' $?$

2) Why do we define $ H_{l}(\mathbb{Q}^{cyc},E[p^{\infty}])$ in the above manner $?$

3) What is the importance of the Iwasawa invariants of elliptic curves i.e what informations do we get about the rank and torsion of elliptic curves from these invariants $?$