How should functional programming be understood?

recently looking at the react-related technology stack, I feel that a lot of things seem a little laborious.
such as the related use of redux-saga, as follows:

import { createStore, combineReducers, applyMiddleware } from "redux";
import userNameReducer from "../username/reducer.js";
import createSagaMiddleware from "redux-saga";       
import rootSaga from "./saga.js";                   

const sagaMiddleware = createSagaMiddleware()        // create/ sagaMiddleware

const reducerAll = {
    userNameReducer: userNameReducer
}


export const store = createStore(
    combineReducers({...reducerAll}), // fn()reducer
    window.devToolsExtension ? window.devToolsExtension() : undefined,    
    applyMiddleware(sagaMiddleware)                 // applyMiddlewaresagaMiddlewarecreateStorefnAfnA({},fnB(fnC))fn()
)

sagaMiddleware.run(rootSaga)                        // sagaMiddlewarerunrunrootSagafnA(fnB)

there are two problems:

import createSagaMiddleware from "redux-saga"; be used directly like deepClone () in import {deepClone} from" lodash";? Instead, you need to return the function to a new function in the form of fnA (fnB), and then use it.
this kind of fnA ({}, fnB (fnC (fnD), {})) this way of writing looks so difficult to understand.

ask the bosses for advice.