Here are a few links worth reading through:
- Long (default) and Short (alternate) Certificate Chains Explained
- Help thread for DST Root CA X3 expiration (September 2021)
- DST Root CA X3 Expiration (September 2021) - Let's Encrypt
- Issue root ISRG Root X3 with RSA cross-signed by ISRG Root X1
You can find more by searching this forum for DST Root CA X3.