Take the standard Hiwin rail, as commonly used in 3D printers:
They come in a max length of 2500 mm. Let's say I need twice that. If I connect two rails together end to end (as a "butt joint") and just bolt them into place, will it work as though it was one 5000 mm rail? Or are we limited to whatever lengths the manufacturers put out.
The answer is yes; and it's a normal thing to do.
https://www.igus.com/info/linear-guides-butting-rails-together
Basically you should only do it with rails from the same manufacturer and you might have to file the mating surfaces, but butting them is altogether trivial. This method works with Drylin and UHMW-PE bearing surfaces as recirculating balls could get caught in the small gap that is left behind. This would cause the carriage to "jump" as it passes over the joint.