1. Introduction  
2. ASR Training and Resources  
3. Multilingual ASR Models  
3.1. Models that Use Only Labeled Data  
3.2. Models that Use Unlabeled Data  
4. Discussion  
4.1. Multilingual vs. Monolingual Models for Low-Resource Languages  
4.2. Techniques and Architectures  
4.3. Performance on High-Resource Languages  
4.4. Factors that Influence Cross-Lingual Transfer  
4.5. Role of SSL and Unlabeled Data  
4.6. Open Problems and Challenges  
5. Conclusion