Is it possible to assign DID/individual numbers to a customer and assign account and password which will enable them connect to this number via any sip phone or app using login and password set on yeti and can then make calls.
Yes
Please where do I read on this and replicate. or how do I go about it
There is nothing special in routing calls to your clients. In this scenario yeti “customer” is your provider, and “vendor” is you client and you have to configure routing as usual - customer auth, routing plan and dialpeers. see Basic configuration — Yeti documentation
Oh what I mean is like the PBX functionality when you set up a DID with SIP interface, User and password which could then be configured on a sip phone and calls initiated just like a telephony functionality
Gateways — Yeti documentation registered aor mode