How do hackers change (mask) IMEI numbers?

Recently, I saw a "Google Pixel 3XL" with a seller, and he claimed that its IMEI number had been changed. When I asked, how did he know that? He said this is because of the "Magisk" app installed. Then he verified it through our official country's website and it was an IMEI of another smartphone, "Sharp Aquos r2." He also mentioned that upon factory resetting, the IMEI number will be lost. So how can the Magisk app change the IMEI number? I think it just masks the IMEI.