The KiwiSaver provider managing your investments doesn't directly touch your money. It goes to an independent custodian who keeps it safe for you. This money is yours and can't be withdrawn by the government, or anyone else.
Based on the type of KiwiSaver fund you're in, the KiwiSaver provider then decides on which assets that money should be invested in. These assets are then acquired and held in trust by the custodian with oversight from a supervisor.
Funds hold various splits of income and growth assets, depending on the category. Income assets are things like bank deposits or bonds, which tend to give a more predictable return. On the flip side, growth assets such as company shares might provide higher returns because they're riskier. The type of fund and tolerance for risk determines the mix of these investments.