I would like integration external pos with erpnext
But I don’t know starting
You need to give us more info. If it is an old system, maybe the easiest thing is to just write directly into database. Or if you can you could consume API.
BTW every doctype has its table in database. Same for API.
POS name erply pos system
ERPNext ver 13 has a great POS. POS Awesome is another option. Please evaluate and check if you can use either of these. Integrating a 3rd Party POS is going to take a lot of time, energies and money.
Any exclusive features of erply pos system? It looks pretty impressive of course!
Hope this helps.