Industrially boeing / airbus are 100k large companies, i.e. need 300m+ base country size (or in EU case, block size) to support specialized workforce large enough for modern commercial aviation. Economically, fuel costs, i.e. engine maturity makes any entrant that's doesn't have parity engine core tech automatically none viable because simply higher costs due to lifetime fuel costs. (Geo)politically, there's lots of certification / safety drama that incumbents like US/EU will throw to undermine competitors so really also matter of geopolitical power - i.e. apart from engines, PRC COMAC using western components mostly for easier certification, they have large enough internal market to sustain development against economics. Almost no one else has those conditions, except India but they don't have industrial base.
Since topic is tankers, PRC/RU has their own tankers, i.e. for military aviation it's not strictly as difficult since fuel cost less issue. But for strategic aviation (transport/tanking) big efficiency working with commercial chassis / turbofan efficiency.
Since topic is tankers, PRC/RU has their own tankers, i.e. for military aviation it's not strictly as difficult since fuel cost less issue. But for strategic aviation (transport/tanking) big efficiency working with commercial chassis / turbofan efficiency.