Net neutrality is about service to last mile customers, but it is based upon interconnection agreements across the L1 and L2 level.

ISP’s pay for a connection to L1 and L2, so their users (who pay ISP’s) can access content on those networks. Websites pay for a connection to L1 and L2 so their content can be available on those networks.

ISP’s want to also charge websites for access into their networks of users, in spite of the fact their users already pay them for access to the website content. If some websites don’t pay, then ISP’s will provide a lower service to their users for those websites. Net neutrality says ISP’s should not do this.

Differentiating between locally used data and hotspot data has nothing to do with this. Hotspot data is about the device the data is going to, not where the data is coming from, and typically (or at least traditionally, maybe not so anymore) a PC will use more data than a phone. A PC is more likely to have large multi-gigabyte downloads (eg games), although these days video streaming is perhaps the main bandwidth hog and is generally equal across all devices.

A home internet connection is expected to serve all devices in that home, while a mobile internet connection is expected to serve only that mobile device (excluding mobile broadband options, which serve multiple devices but are typically more expensive). The ISP’s network is designed with this in mind.

It is more reasonable for an ISP to only provide data to the phone you’re paying for than it is for them to throttle websites you already paid for. However, really both are kind of bullshit - usage limits in general are completely disproportionate to actual costs.