In other words, businesses in older buildings did not have to rip out narrow doorways, stepped floors, and tiny bathrooms until or unless they renovated.
I could be wrong, but since the ADA was a federal law, requirements for compliance are probably the same in all states.
Yes they are. States can write their own versions of ADA if the standards are stricter than the federal one. For example Florida requires a lavatory in H/C access bathroom stalls while the Fed regulations do not.