LightningModal can be used in the Salesforce Lightning Web Component for Modal Pop Up window.

Sample Code:

Modal Lightning Web Component(Name:popUpComponent):

HTML:

<template>
    <lightning-modal-header 
        label="Heading">        
    </lightning-modal-header>
    <lightning-modal-body> 
        {content}
    </lightning-modal-body>
    <lightning-modal-footer>
        <lightning-button 
            label="Close"
            onclick={handleClose}>
        </lightning-button>
    </lightning-modal-footer>
</template>

JavaScript:

import { api } from 'lwc';
import LightningModal from 'lightning/modal';

export default class PopUpComponent extends LightningModal  {
    
    @api content;

    handleClose() {

        this.close(
            'Closed'
        );
        
    }

}

js-meta.xml:

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
    <apiVersion>58.0</apiVersion>
    <isExposed>true</isExposed>
</LightningComponentBundle>

Parent Lightning Web Component:

HTML:

<template>
    <lightning-card>
        <lightning-button 
            class="slds-m-around_medium"
            onclick={showPopUp} 
            label="Show PopUp">
        </lightning-button>
    </lightning-card>   
</template>

JavaScript:

import { LightningElement } from 'lwc';
import popUpComponent from 'c/popUpComponent';

export default class sampleLightningWebComponent extends LightningElement {

    async showPopUp() {

        const result = await popUpComponent.open( {
            size: 'large',
            description: 'Popup Dialog',
            content: 'Testing Content from Parent Component',
        } );
        console.log(
            result
        );

    }

}

js-meta.xml:

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
    <apiVersion>59.0</apiVersion>
    <isExposed>true</isExposed>
    <targets>
        <target>lightning__Tab</target>
    </targets>
</LightningComponentBundle>

Output: