Implant Specialist Alex Rugh explains what UCLA abutments are and how they work. They can be used to make traditional screwretained PFM or FCG crowns as well as custom abutments.
The UCLA technique was the only way to fabricate custom abutments before CAD/CAM abutments became available. These days we only use this technique when a CAD/CAM abutment is not available.

UCLA Abutments are available for just about every implant system including:
Nobel Biocare Conical Connection
Straumann Bone Level, Tissue Level, BLX and TLX
Dentsply Sirona Astra Tx and Astra EV
BioHorizons
Zimvie TSV & 3i Certain
Implant Direct