do ringworlds excist in SE5?

Yes. You need to research Stellar Manipulation to lvl 70.

You reach level 70 Stellar Manipulation by researching it through the research tree. Once you get the tech, you build the components in Baseships and Starbases around a star. The star has to have a significant clearance of emptiness as the Ringworlds and Sphereworlds are quite large.

If you don't feel like waiting until Tech 70 and are feeling impatient, you could use the alltech cheat to get all technologies.

In game in the System View, press the tilde key ( ~ ) on the upper left part of the keyboard. Type in ' alltech ', and press the ~ key again. Your empire now has all tech levels in the game. This will give you access to use the Ringworld and Sphereworld components.

Yes stock game large quadrent went on a major tech rush took about 30 game years to build the first ring world in a former nebulae system, after building the sun for that system

To make a baseship go to the design window, click 'create new', the double click where it says 'Baseship'
As for creating the star, there's a component called 'Stellar Plasma Sphere' that you can put on the baseship to make a star
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an unoriginal signature

1a base ship is the largest avalible ship hull which requires you to have developed Starbase tech to tl 5 to get as well as huge hulls (dreadnoughts).

Stars are built using stela manipulation tech which gives you the component to do it. I cant recall the tl but its lower than ring or sphere world building requires.

It also alows you to open and close warp points create and destroy nebulae and black holes create and destroy storms, build planets from asteroid belts and destroy them all at various TL's.

All this is great fun but the AI is not yet up to using it.

If you have all the techs in stellar manipulation, you might want to try making a sphereworld instead of a ringworld (sphereworlds have twice the facility, population, and cargo space of a ringworld).
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an unoriginal signature

You can't make ringworlds out of asteriod belts or planets, you can only build them around stars using the stellar manipulation components Ringworld Placement Generator, Sphereworld Placement Generator, Hyper-density Cables, and Gravity Plating.

you cant make em out of planets or stuff.

Ringworlds and sphereworlds are ridiculously, excessively, unnecessarily huge and productive, but the research cost is... prohibitive, and that's being generous. Stellar Manipulation is horribly impractical in this game.

How do you think it will be in reality?

I think the maker of this game has alot of things that would be real in reality as to how hard it would be to build them research them etc.

While the tech requirements are ridiculously high in SE5, they're better than SE4.

In SE4, I found it often easier (if the computers started off right next to me and in hostile moods) to research to where I could collapse wormholes, then turtle in my home system and maybe 2 or 3 others and build up tech to where I could build ringworlds and/or sphereworlds.

If I built more than one of those, no matter how insignificant my empire was in any other regard, I usually tipped past the Mega Evil Empire point on the scoreboard.

So far in SE5, I haven't gotten to where I can even make a planet. The tech tree is far larger than the game's playable area. By the time you can make a planet, you own all the existing ones and there really is no point in actually building your own.

This all refers to stock, of course.

You can easily edit the cost and level requirements in the components.txt file.