Everything available via “pip install dash” you can use without limit. I believe all the open source Dash projects are under the MIT licence which is a very liberal commercially friendly licence.
However you can pay for Dash to get access to extra tools and direct support from Plotly and paid for feature requests. Such tools including deployment server, enterprise authorization integration, a Dashboard creator etc…
This is Plotly’s business model, put awesome free open source libraries out there and if you want commercial support or professional tools to go with them then you can pay for that.
Unfortunately the front page of Dash pushes this business model pretty aggressively and, like you, I’ve had lots of people say when I tell them to try Dash “oh, don’t you have to pay for that?”. But you don’t, the library on it’s own is great without any paid extras.