Various

What is a PWA application?
PWA, or Progressive Web App, is a technology that allows customers to install a full-fledged mobile application on their smartphone without the need to publish the app in the App Store or Google Play. 
Now there is no need to develop separate applications for iOS and Android. It is enough to have and maintain a «progressive web application».
Mobile applications based on PWA are used more often than it seems at first glance. Twitter, Tinder, Uber, Telegram, Starbucks, Forbes, AliExpress, and Aviasales use PWA-based applications as a foundation or as a supplement to their mobile application.
Advantages of a PWA Application
Development and maintenance are 70% cheaper than for mobile applications.
The technology allows sending push notifications.
Savings related to customer re-engagement, as there are no costs associated with customer return tools (retargeting, remarketing).
No need to publish the application in the App Store and Google Play. You can download it directly from the website.
Works significantly faster than a website.
Works without the internet.
The size of a PWA application is usually less than 1 MB, which is smaller than a mobile application.
The PWA application works with the following features: geolocation, camera, microphone.
What tasks does PWA solve?
Drive repeat sales.
Make it easier to find your company. Get access to the company with 1 tap on the screen.
Increase brand awareness. The company symbol (logo) is always visible to the user.
Increase your average order value by boosting product sales through push notifications.
Stay connected with the customer. Send the customer notifications about product arrival at the time of pickup. Or that the product will be delivered today.
Save on advertising. You no longer need to set up retargeting ads with a new advertising campaign. Just send a notification that your customers are sure to notice.
Eliminate competition in search results. Your customers connect through an application that has no competitors, unlike search results.
Additionally
PWA can be downloaded and promoted on Google Play.
PWA can be created for a part of the website.
The application is updated using cache, i.e., every time the user opens the application with the internet enabled.

What is Bootstrap?
Bootstrap is the most popular HTML, CSS, and JS framework for developing mobile-first projects.
Bootstrap is the most used framework in various projects around the world. Its closest competitor has a community several times smaller. In Bootstrap ready-made CSS styles and JS scripts are provided, to apply which you just need to specify the necessary style classes and attributes for HTML elements.
Bootstrap is primarily known for its excellent responsive properties for working on mobile projects. All thanks to the famous Bootstrap grid, which allows you to easily adapt any website and display it well on any device.
What is Bootstrap used for?
In general, to better understand why you need Bootstrap, we can go back a bit and answer the question: “What is a CSS framework in general?”
Essentially, in simple terms, it is a file or several files with ready-made code that are connected to the website in the head section, after which you gain the ability to use the features of this framework.
Frameworks are created to make it easier for other web developers to build websites. I already mentioned at the beginning that today almost any developer, having built a couple of websites from scratch, thinks about how to speed up the development process.
The fact is that if we are going to develop a website from scratch, we will have to take care of many things. All CSS styles and web scripts will have to be written from scratch, which can amount to hundreds or thousands of lines of code. Moreover, you can make a lot of mistakes in the layout. For example, your template will simply look different in major browsers or will not respond to queries.
In general, just for the sake of a responsive layout, you should use Bootstrap, because if we are talking about fixed layouts, they are easy to create even from scratch. We just create blocks, give them a fixed width, and work with the layout.
But in the case of responsive layout, everything is much more complicated. You will need to ensure that your website displays correctly at any screen resolution. For this, you will need to use media queries. For larger projects, you may need many such queries, plus you will still have to learn how to write them.
In general, when you develop a responsive layout from scratch, you will have to work hard, and your layout skills must be quite high.
But what about Bootstrap? If you learn this framework, it will significantly simplify the layout for you. First of all, the framework takes care of cross-browser compatibility and performance, which are the main things a developer should care about. But with Bootstrap, they are very easy to implement. This allows creating an HTML template even for a person who has previously done very little layout work and is not particularly familiar with CSS.
Secondly, the structure is ideal for teamwork. With the right skills and understanding, coding in Bootstrap is 3-5 times faster, and code consistency will allow any of your colleagues to make changes. If we are talking about a layout without a framework, each developer may have their own style, and someone else will have to spend some time studying their code.
As an advantage of the structure, I would also like to highlight the large Russian-speaking community and the availability of good documentation in our language. Thanks to such prevalence, many templates have appeared for Bootstrap, where the design of all main elements has already been reworked. You can connect these templates and build your websites based on them, just changing something slightly.