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 General 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 General 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.