The Source Novel answers the ending, the drama doesn’t (apparently they prolly had a plan for S2 which never happened)

The present day Yai at the ending scene of Ep 12 is a reincarnation of Khun Yai, His name is Yai Kanthorn. Khun Yai of 1920s was also a reincarnation of Commander Yai (the mustache Yai of the drama).

Khun Yai didn't get married in his lifetime after Jom left the era. Khun Yai's younger brother Lek got married & had 2 daughters. Lek's youngest daughter Uaem Duaen adopted her elder sister's son, which is this Yai Kanthorn. So He is Lek's grandson & technically a descendant of Khun Yai. Khun Yai gave all his properties to his favourite niece Uaem Duaen, & She promised him that she will search for Jom & keep the small house as it is. In the present timeline, Uaem Duaen found Jom in a magazine/portfolio & showed his picture to Yai Kanthorn (also 'specifically' asked the agency for Jom as the architect of her house renovation) . After seeing Jom's picture shown by his mom, Yai Kanthorn started having dreams of his past life. By the time Jom finished time travelling & was back in the present, Yai Kanthorn already had acquired all his past lives' memories (including Commander Yai & Khun Yai). The last scene of the drama is where they officially met for the first time. It's a Happy ending for them in this Lifetime.

There is only one Jom, but 3 Yai (19th century-Commander Yai, 20th century- Khun Yai, 21st century- Yai Kanthorn). Jom time travelled twice in the Novel, first from 2023 to 1920s, secondly From 1920s to 1800s and then returned to his original present timeline from there. In the drama, when Khun Yai wants to go to France for studying & shortly after he parted ways with Jom, Jom disappears from 1920s but time travels back in 1800s era (Ending Credits scene). There he met the first incarnation/First lifetime of Yai which is Commander Yai. This is the Era where karmic cycle of the Main characters started.