Very few diagrams of the solar system are strictly accurate for a reasons.

1. The relative size of the planets + the sun are just too different from each other. The sun is 285 times as wide as Mercury, the smallest of the 8 (It should be 9, I still love you Pluto) major planets.
2. The distances between them are massive and if drawn to scale would never fit on a single page while still allowing you to see all of the orbits.
3. Most diagrams are 2 dimensional representations and the planets actually orbit in 3 dimensions.

While the major planets (and many other solar system objects) orbit the sun in roughly the same disc they aren’t all doing so on the exact same plane.

Put another way of you were to look at our solar system from the “side” such at the earth stayed “flat” as it rotated around the sun, ie it never appeared to move up or down, the other planets would NOT do the same. As they orbited they would appear to go above and below the line drawn between the earth and the sun by as much as 7° in the case of Mercury. The Earth-Sun orbital plane is called the ecliptic and it’s one reference point we use when talking about orbits, because we live on Earth so it’s convenient to do so. But it’s not the ONLY reference plane you can use.

For example, you might already know that the Earth is tilted relative to its orbit around the Sun, but did you know the Sun is tilted too? I’d we drew our plane relative to the suns equator instead of the sun Earth orbit, earth would appear to move above and below this imaginary plane by more than 7°! In fact Mercury, the planet with the greatest variation relative to the Earth/Sun plane (the ecliptic) is the one that varies the least relative to the Suns equatorial plane.

There are other reference planes too. You can make one based on the orbit of any object around the sun. Or you can find the average of all the planets and weight it relative to their mass. That particular plane is the Laplace invariable plane.

So why don’t they all orbit on the same plane? Because planetary system formation is messy! Everything (well almost everything) in our solar system started out as big dust cloud. Not everything was moving in the same direction but overtime as objects collided and gravity pulled things together their average radial velocity ended up in roughly the same direction. But roughly in solar system scales leaves a bit of room for variation. Collisions over time perturbed the orbits (and rotational angles) of different planetary bodies.

Take Uranus for example. It’s spinning basically on its side (90°) relative to its orbit! That means one of its poles points roughly in the direction of the sun.

Meanwhile Venus is spinning backwards relative to the other planets, which probably means it got flipped over at one point due to one or more collisions.

And on top of all that, despite what it may look like the planets don’t orbit in perfect circles either but in ovals. Earths distance from the sun varies between 147 and 152 million km over one orbit (one year). Little Mercury varies between 48 and 70 million KM in its rapid orbit (88 earth days). And poor demoted Pluto has an even greater orbital variation over its epic 90,560 day “year” coming as close as 4.4 billion km and getting as far away as 7.4 billion km. Meaning during part of its orbit it’s actually closer than Neptune. This lasts about 20 years and the recent recent occurrence was from Feb 1979 to Feb 1999. Unfortunately you’ll have to wait for about 230 years for it to happen again, so if you were born after Feb 11, 1999 you missed it.