> Russia has built many large aircraft in their Soviet days, but their industrial base has eroded so they probably just don't have the capacity to build competitively-priced aircraft in volume now, plus they're not well-liked by most countries that would want airliners anyway
Russia did design and build a few designs recently - SSJ-100, MS-21, Tu-204/214. Almost all of them used Western parts (such as engines/avionics) to speed the process up because Russian manufacturers are a few decades behind in some critical parts (like engine efficiency). That being said, they're currently trying to ramp up production on a domestic only Tu-204/214 version, but it's slow, probably quite expensive, and super inefficient.
> Japan might be able to do it, but just isn't geared up for building lots of large aircraft so it would be a huge investment to build such an industry almost from scratch.
Mitsubishi had a regional jet design in progress, the SpaceJet, which got cancelled after a few billion in R&D and years in delays.
Ukraine also had decent manufacturing facilities and capabilities with Antonov and Motor Sich, including decent recent designs and subcontracting work for other companies.. but with the war a lot of things were destroyed (like Antonov and Motor Sich factories and facilities), and their focus is mostly on drones and missiles now.
> Fuel efficiency isn't a factor because building an airliner doesn't involve building engines for that airliner, and engines determine an aircraft's fuel economy for the most part. Boeing and Airbus don't make jet engines: those are all made by Rolls-Royce, Pratt & Whitney, and GE.
Fuel efficiency is a major factor. Outside of countries big enough to prop up a domestic airliner industry with enough spare cash to compensate for inefficiency (which is a list of 1 country, China), nobody will buy a jet which is less efficient than an alternative. Airline margins are razor thin.
And not everyone can buy an engine from Rolls-Royce, GE/CFM(their joint venture with Safran that designs manufactures short-haul engines and the future open fan design) and Pratt and Whitney. The Russian UAC cannot. The Chinese COMAC got sold a decade+ old variant for their latest jet out of fear of industrial espionage (the Chinese engine manufacturers have failed at producing reliable and efficient jet engines for any application, usually relying on Russian designs).
TL;DR is that modern airliners are extremely complicated, expensive, and have very long lead times. A new design won't be making any money within the first 10-15 years of its life, which means that only countries with very developed and advanced industrial bases, lots of money, and the desire for strategic airliner autonomy will invest in them.