Is it a two way process, if we do DirSync in order to sync users from AD and make any changes in tenant admin center to the synced user [some properties], will the changes reflect in AD too. (or) is it just one way process ? [ assume that the domain is added]
With DirSync, no. With Azure AD Connect it is possible to do writeback for passwords and devices (requires Azure AD Premium subscription) but it does not do full writeback allowing updating full set of attributes for user or devices. Such feature apparently was in Preview at some point but not generally available.