What I mean is... many cases are individual: the ramp for that library; the door knobs/handles in that office; the floor indicators in this lift/elevator; the audio announcements (or lack of) on the busses/trains; the small print on contracts; live subtitling at the theatre; the checkout button on this web site; the checkout button on that website. You get the idea.
The vast majority of cases are individual; owned by myriad businesses, charities, foundations.
The problem the OP is circling is the same as that proposed by the Social Model of disability. Most systems present obstacles to disabled users which could have been avoided if those systems' owners and designers and included disabled people in the design phase. Instead, no-one does this claiming it's too expensive, forgetting they themselves will be disabled in the future (most people become disabled in one way or another, with age). We all kick the can down the road, rather than deal with it today. Dealing with it is hard. The "national level infrastructure" aspect is a mixture of cultural praxis/expectation and, potentially, government carrot to go with the (existing) government stick (regulation; that most people try to ignore).
(I'm not saying the Social Model of Disability is 100% right. I feel it's viewpoint is hugely instructive it's not the whole story.)