'Hope you are all doing well' is the kind of thing you put in an informal letter/note/email to friends or family. It's usually a very casual/catch-all question you might put at the beginning of the conversation - the person writing is not really enquiring after your health or financial state!
e.g. Dear Bob and Sally - how is everything with you? 'Hope you are all doing well?'
It can be a way to sign off too.
e.g....'Hope you are all doing well' See you soon, Love Martha x
The response should be just as casual e.g.
'Hi Martha, we are all well, thanks' You could add more detail e.g.
'......We are all well, thanks - the twins are getting so tall you wouldn't recognise them!'
If things are not well - and you feel it's appropriate you can always share some more information, but you don't have to.
e.g. 'I'm sorry to say things are not so good at the moment. John has been made redundant and things are very hard. To make matters worse the twins have had chickenpox!'
It really depends on your relationship and what you want to share.
