If your QMS is already compliant to the ISO 9001:2015 standard, then it becomes easy to integrate the ISO 14001:2015 standard for an EMS, or even the ISO 45001:2018 standard for an OHSMS. All of these standards follow a common format, so it is easy to go through an see which requirements are common, meaning that you can use a common process. You will need to perform a gap analysis to the additional standard to see what additional requirements you need to add.
The most common processes to be easily integrated include Internal Audits, Corrective Actions and Management Review. However, you can also reuse many processes such as the identification of interested parties and their needs and expectations. If you have already done this for your QMS, you just need to use the same process and re-think who the interested parties are for your EMS or OHSMS.