How to Run/maintain "one" CI3 App hosted on multiple subdomains/ servers.

you can have different controllers for differest clients like
routes --> client1 go to --> client1/home
routes --> client2 go to --> client2/home
i dont know how complicated is your system but with routes you can redirect everything,

about the dbs info in 1 db i use like aes-256-cbc with iv password etc and i encrypt/decrypt when i want.
Btw you can keep the database connections in database file ( all of them ) and according to the site to load the info (i do this in models, so the controller will be clear you just pass the name on the db connection)


If the project is like CRM which the clients must login to do what ever they want, just save in ccokie the client name and keep the routes file
clean.
hmm maybe you could save the client name on 1st entry in cookie and do the rest of the staff according to that(encrypted ofc)