Most of the time there's a gap under most internal doors to allow for airflow between spaces. Most rooms also have ventilation and return air, but often a gap under the doors as well.
It's normally not very much, but enough for an ethernet cable. The trick is to get it to sit neatly across the area so it doesn't impede the swing of the door.
I like to use small nails or screws in the frame of the door to hold it against the flooring.
The hardest part for me to remove when departing my previous rental was pulling down the conduit. I didn't use the 3M command strips for it, since it came with mounting tape pre-applied. I pulled off some of the layers of paint on the wall when it came down. It was not a big deal to the superintendent since they repaint regardless. The cup hooks took the longest since you have to untwist each from the wall by hand. They're not fully enclosed, so pulling the cable out was simply a matter of pushing it up and out of the hook.
When done correctly, it should only take about 20 minutes to pull apart and leave little more than paint damage behind.
If that's not appealing, and you have cable TV or some other kind of coaxial lines run between rooms, there's MoCA as an option. It can coexist with most TV signals, and something to consider if you're in a situation where you have the wiring needed. If only phone lines exist, it's entirely possible to buy and use ethernet extenders that use DSL technology to relay the data. They're not as fast as MoCA, and they may not even have as much bandwidth as wireless, depending on the conditions, but they will be far more consistent and reliable than wireless.
There is the option of powerline adapters as well, however, I would only recommend them if you have enough knowledge of the power lines in your home to determine that the plugs that the powerline adapters will be placed into are on the same circuit. If they're not, then the situation can become very complex, or downright guesswork to try to get working. It becomes a huge risk for anyone who isn't an electrician with enough knowledge to determine if the lines will intersect in a productive way. Here in North America where I am, we use split phase power, so if powerline adapters are used here and they end up on different hot lines, the signal essentially has to travel through either the transformer that's delivering the power, or the neutral lines which may be afflicted with all sorts of interference from the ground being bonded to it. So if you're not an electrician, I would say only to use powerline if the plugs you intend to put them on are on the same circuit. If not, you're going to have significant risk for the system not working well, reliably, or possibly even have it not working at all.
I'm certain there are other options I haven't really gotten into, but there's plenty out there to use and try. Ethernet or fiber is generally ideal. If you can't use that, then MoCA, and to a lesser extent DSL. If that's not an option for whatever reason and you are in favorable conditions, maybe powerline. Beyond that, you're stuck with wireless. Ethernet/fiber can run upwards of 100Gbps (SMF) to 10Gbps (MMF/Cat6), or mgig at 5Gbps or 2.5Gbps, and of course 1Gbps. MoCA can do an excess of 1Gbps last I checked, but lacks full duplex operation like most ethernet standards. DSL can be as fast as 250+Mbps with the right equipment, but often trends around 50-150Mbps in the simple point to point configurations you'll find with non-ISP grade solutions. Powerline wildly fluctuates depending on conditions, but can achieve 1Gbps in ideal situations. Wireless, which by its very nature, is half duplex, is by far the most variant, if you're the only person using it in the local area, it can be fantastic, however, such an ideal is extraordinarily rare, it interferes with everything from wireless console controllers, microwave ovens, neighbors and just about everything else made for consumers with a wireless connection. Often, it's the most variant of the bunch and frequently has the longest ping times and jitter.
Simply put, wireless is a bad technology. Not because it was/is done poorly, but because it works so well that everyone puts everything onto it and that makes it terrible. There's only so much wireless frequency range given out for unlicensed public use and when everyone has their own wifi, we all end up stepping on eachothers toes.
My motto is, and has been for a while: wire when you can, wireless when you have to. If everyone did that, we would be much better off.
