Biogas vs natural gas
Same core molecule, very different purity and origin.
Natural gas is mostly methane extracted from geological formations. Biogas is methane produced by bacteria from recent organic material, mixed with a substantial fraction of carbon dioxide and trace contaminants.
Upgraded biomethane is functionally interchangeable with natural gas and can be injected into the same networks or used in the same vehicles.
The difference that matters practically is cleanup. Raw biogas contains moisture and hydrogen sulphide that will corrode equipment designed for pipeline gas.
Educational overview only. Before acting on any of this, review the safety fundamentals and local regulations.