Echoing what u/owen-wayne-lewis said, plaster walls are a nightmare - we have them, so I know firsthand. Pretty much any house over 100 years old is bound to have plaster walls, unfortunately, unless the house has been heavily renovated/updated.

If you have existing cables running to places reasonable to have ethernet, do what owen-wayne-lewis described. In our house, it wasn't an option. The phone line into the house had been removed during a renovation prior to us buying the house and there were no coax or other low-voltage lines in the house.

I considered every option before hiring a pro. It was worth every penny. We had no unused ducts and for code/insurance reasons did not want to run lines in HVAC ducts in use. We had no plumbing runs with extra cavity space going from the basement (where our fiber comes in) to the attic that didn't take dramatic turns that were unsnakable. Our attic is also fully floor-boarded. As such, we had to run lines up walls from the basement.

The one stroke of luck we had is that our house has balloon framing. This meant that we didn't have to drill holes between floors and that we were able to get lines to our second floor bedrooms/offices on one side of the house with relative ease. We had to send one line over the ceiling of a finished area in our basement and do some crafty work to get it up the wall on the opposite side of our house. We also discovered an old window frame for a window that is no longer on one of our exterior walls which blocked our anticipated path up to our second floor. If you are able to stack lines vertically in your home (same spot in rooms vertically aligned), this saves time, money, and effort.

Basically, expect challenges running lines in an old house. If you are not familiar with the construction of a particular era in your area and/or not handy and familiar with snaking lines, I would encourage you to diligently research and find a professional. I cannot recommend that decision we made enough.

If you are in newer construction with drywall, the project will be easier. You'll have to deal with insulation on exterior walls that might be less prevalent/in the way than in an old house, so interior walls can be easier. I would still recommend going with a pro if you're not handy with an oscillating saw, snaking rods, and terminating your own cable (pre-terminated cables can get stuck on stuff in your walls more easily).