My partner and I have been talking about making some cat paths in our house since long before we owned our house. We recently got 2 new dogs, so now it is a high priority project. The first step was putting up some shelves in the hallway and making a portal from the hallway to the bedroom closet (you can see it just above the tent). The surround for the portal is this 3D printed tunnel
https://www.printables.com/model/3068-cat-door
The next project is going to be putting a portal from the spare bedroom into the stairwell so they can go up and down floors without the dogs being in the way
The dogs don’t really pay attention to them. We put them above their head level, so unless there is a cat on the shelf, they just walk in past. Even then they are not too curious about them, which for us is a good thing. One of our cats (the closer one in that pic) is the biggest scaredy cat, so the shelves and portals have really helped him with being more confident around the new dogs.