What is the difference between OMNI channel and MULTI channel?
It all depends on whom you are talking to. Technically it is totally the same.
Please refer to What is a multichannel PWA article, but in essence it is providing service through several channels at once, for example through sms and email and voice, or mobile app, messengers and sms.
When you look from a company's stand point they want not to care where from a customer comes in for a service, they want and OMNIpotent single system which covers everything, but is a one place for the company.
When you check user's view point, there is a number of channels to get the service from a company. User makes a selection from a MULTItude of choices.