Wacom brought EMR technology to drawing tablets decades ago. In the process they held the original patents (long since expired) and have many new ones.
Wacom lists all their patents here: https://www.wacom.com/en-us/patents
If you have time for some technical reading, these patents below describe some key aspects of EMR technology.
US Patent 4,878,553 - POSITION DETECTING APPARATUS
Date Filed: 1987/09/14
Date of Patent: 1989/11/07
Status: EXPIRED
US Patent 4,999,461 - COORDINATES INPUT APPARATUS (LINK TO PATENT)
Date Filed: 1989/05/19
Date of Patent: 1991/03/12
Status: EXPIRED
US Patent 5,134,689 COORDINATE INPUT SYSTEM AND INPUT IMPLEMENT USED IN THE SYSTEM
Date Filed: 1988/08/24
Date of Patent: 1992/07/28
US Patent 5,691,513 SCANNING METHOD FOR SENSOR COILS IN COORDINATE DATA INPUT DEVICE
Date Filed: 1995/09/25
Date of Patent: 1997/11/25
Status: EXPIRED
US Patent Reissue 4,786,765 COORDINATES INPUT SYSTEM
Date Filed: 1987/07/23
Date of Patent: 1988/11/22
Status: Ceased
US Patent Reissue 5,023,408 ELECTRONIC BLACKBOARD AND ACCESSORIES SUCH AS WRITING TOOLS
Date Filed: 1989/06/13
Date of Patent: 1991/06/11
Status: Ceased