Is LiteSpeed included, or is it an upgrade

On shared SEO hosting and managed WordPress, it is included at no extra cost. It is simply what serves your sites there: no tier adds it and there is no box to tick. On a VPS or a bare metal server the answer is different, because those are unmanaged machines you hold root on: a LiteSpeed licence is an add-on you choose in the order form, the same way cPanel and CloudLinux are.

Why it is the default rather than an option

Think about what a portfolio of sites looks like to a server. Not one busy site, but forty quiet ones, plus search engine crawlers wandering through several of your domains within the same few seconds. Modest total volume, scattered and unpredictable arrival.

Servers that assign a dedicated worker to every open connection handle that badly, because most of those connections spend most of their life waiting on a slow phone network rather than on anything you control. Handling them without tying up a worker each is the entire benefit, and selling that as an upgrade would mean shipping the wrong default to the people who need it most.

Do I have to change anything?

No. Rewrite rules, directory structure and panel behaviour are all as you expect. Sites arriving from an Apache host keep their configuration, which is usually the thing that puts people off changing web server at all.

Will it improve my rankings?

Not directly, and treat anyone who claims otherwise with suspicion. Search engines do not award points for infrastructure choices. They do notice how quickly pages answer, and speed is worth pursuing on its own merits rather than as a trick.

The bigger lever

For a content site the cache matters more than the server. A stored page skips your application completely, which saves far more than any difference between two web servers doing identical work. So the honest ranking of priorities is: cache first, everything else after.

Is it the same on every plan?

Yes. Shared accounts, WordPress plans and servers all use it, so a site behaves the same way whichever product it sits on and moving between them changes nothing about how requests are served.

What about my existing caching plugin?

You can keep it, but it will be doing a slower version of something the server already handles. Most people swap it out after arriving, which has the side benefit of removing a block of rewrite rules that nobody will be able to explain in a year.

Does it change my logs or tooling?

No. Access and error logs sit where you expect, and the panel tools behave identically. Nothing in your routine has to change.

How that layer works is covered in Speed and Caching. Product documentation is at docs.litespeedtech.com.