Building a UserOperation can involve several complex steps.
For an interactive experience, check out the ERC-4337 examples repo.
The context field instructs the paymaster on how you would like your gas to be sponsored. The Stackup Paymaster currently has two types.
payg: Paymaster sponsors the gas and invoices the fee in USD to your monthly subscription.
erc20token: Paymaster sponsors the gas and pulls enough tokens from the sender account to cover the cost.
For more details see the guide on Paymaster RPC.