They're just standard Lightning nodes that use Keysend rather than Lightning Address/LNURL/Invoices to receive payments. Most of them (other than Fountain) are self-hosted and self-custodied rather than being hosted by a custodial wallet.

Both Keysend and LNURL are ways to get around Lighting's requirement that a sender needs to ask the recipient to generate an invoice before they can pay them. There's no way to send a "spontaneous payment" in standard Lightning.

Keysend works by sending the payment with an empty invoice. The recipient's node just receives the sats and ignores the fact that there's no invoice attached. The sender uses the node's public key to send the recipient sats via the Lightning network.

LNURL works by putting a publicly-accessible webserver in front of the recipient's node who's only job is to generate invoices for people who ask for them. The sender uses the recipient's Lightning Address to look up the webserver, sends a request to the LNURL callback, and gets an invoice back. They then can pay that invoice on the Lightning network.

V4V nodes chose keysend because it's simpler and requires no additional lookups or servers in the middle of the payment process. You just send a payment from your node to the recipient's node through the Lightning network and that's it.

Wallet providers, including most of the ones the Nostr folks use, chose LNURL and ignored keysend entirely, which is why many here are unfamiliar with it.