How do we know what our galaxy looks like when the furthest we can see os pluto

Plus, how do we know where the centre of the milkyway is if we have never been outside of it to look at it

With visible light telescopes we can see other galaxies of many types, sizes. With radio telescopes we can chart the gas, dust and active regions of over 3/4 of our own galaxy. From that, we can see its size, where its center is, and the shape of the spiral arms.
From comparisons of the two kinds of images, we can be fairly sure that our galaxy is quite similar to many large spiral arm galaxies. A surprise a decade or so ago was the new idea that the center has a short bar across its center, rather than being a plain spiral. We did not see it before because the bar is pointed almost at our location.

The most distant thing you can see with the naked eye (which cannot see Pluto) is the Andromeda Galaxy at 2.3 million lightyears. Tellescopes can see things out to about 12 billion lightyears. Where did you ever get the weird notion we can't see anything past Pluto?
Studies of the structure of our galaxy began with Herschel a couple hundred years ago. Edwin Hubble and H. D. Curtis were two of the main people responsible for proving where the center of our galaxy is. From there studies of the distribution of stars, nebulas and other material showed that ours is a spiral galaxy, with us located on the outer edge of the second spiral arm out.

The furthest we can see is not Pluto. What made you think that? True, Pluto is pretty much the boundary of our solar system, but it's not boundary of the visible universe. Look up at the sky at night. Those twinkling stars are not planets. They're stars!

There are these things called telescopes. I'm sure you've heard of them. The ones normally used to look at distant solar systems and galaxies are not the kind you set up right outside your window. Astronomers use powerful observatories with unique lenses and huge mirrors to view the cosmos. Stars are bright and are therefore pretty easy to point out. The planets are the tricky ones. To detect planets outside of our system, astronomers detect slight changes in the brightness of a star as a planet passes in front of it.