Which mobile networks does an eSIM use in Japan?

Japan has 4 networks a roaming handset can register on (NTT DOCOMO, KDDI (au), SoftBank, Rakuten Mobile), running 4G/LTE/5G. The phone picks whichever mast is strongest where you are standing rather than staying tied to one of them.

You can switch to manual network selection in the handset's settings, but in most cases you should not: automatic selection is what moves you to a stronger mast as you travel. If one specific spot is unusually bad, picking a network by hand there is a reasonable temporary fix. Set it back to automatic once you leave.

Japan has live 5G, and a handset that supports it will usually show 5G in urban areas and drop back to 4G outside them, which is still fast enough for maps, messaging and a video call. Whether a given mast serves 5G to a roaming line is a separate question from whether 5G exists in the country.

The name on your screen is the local network your handset is registered to at that moment, and it can change during the day as you move. That is how roaming works and does not mean the plan is wrong. What matters is that you have signal and data, not which operator name appears.

The pattern is the same almost everywhere: the network follows people and roads. Cities and main routes are solid, and coverage thins in mountains, forest, open water and minor roads, in proportion to how few people live there, not to how far from the capital you are. Download an offline map of any stretch you plan to drive outside the cities.