Networks
Every database (wallet file) belongs to one network, chosen when it is created. By default, Zkool creates mainnet wallets.
In the Database Manager, tap the + button. The Create New Database dialog has a Network dropdown with Mainnet, Testnet, Regtest and ZSA Regtest. It starts on the network of the wallet currently open; pick another one for the new wallet.
The network is stored in the wallet file and cannot be changed afterwards: to move to another network, create a new database and open it.
Tips
Older wallets were put on testnet or regtest by their file name (testnet or regtest in the name). They keep the network they were created on; the name no longer matters.
Server and explorer
A new wallet starts with a server of its network: https://zec.rocks on mainnet and https://testnet.zec.rocks on testnet. On the regtests, it points at a local lightwalletd (http://localhost:8137) or at the ZSA testnet server; change it in the Settings page to match your setup.
The server field of the Settings page has a button that opens a list of known servers of the wallet's network, with their status.
The block explorer link of the transaction page follows the network too: zecblock.com on mainnet and testnet.zecblock.com on testnet. The regtests have no explorer and show no link.