1. Introduction  
2. Data  
2.1. LOFAR Two-Metre Sky Survey  
2.2. Ancillary Data  
3. Methodology  
3.1. Equipartition Magnetic Field Strength  
3.2. Thermal Emission  
3.3. Correlations  
3.4. Accounting for the Influence of Cosmic-Ray Transport  
4. Results  
4.1. Magnetic Field Strength and Scale Length  
4.2. Magnetic Field–Star Formation Relation  
4.3. Magnetic Field–Gas Relation  
4.4. Influence of Cosmic-Ray Transport  
4.5. Kinetic Energy Density  
5. Discussion  
5.1. What Regulates Magnetic Fields in Galaxies?  
5.2. Limitations of the Equipartition Approach  
6. Conclusions