You can call a microflow in ‘Default home page’ or ‘Role-based home pages’ depending on your requirements. These will be triggered as soon as the page is loaded. If user is not logged in then login page will be displayed else default or role-based microflow(s) will be triggered straight after login as well as whenever page is loaded.
If you want to only trigger a microflow just after sign in and not when everytime page is loaded / refreshed then use SignIn microflow for Mx7 / Mx8 module from app store.
Hope this helps!