There are two things you can do right now
In April 2020, Japan made it possible for foreign trainees to find jobs in industries other than the ones they had trained in. This was because a lot of foreign trainees were stuck in Japan due to the covid pandemic and travel restrictions. Many also lost their jobs so couldn't afford airfare. This change in the immigration policy is a huge relief
It is also possible now to convert your current Technical Intern Training Program visa or student visa to Specified Skilled Worker visa (Tokutei gino). But for this you must have stayed in Japan for more than 2 years and should be currently in Japan. Here are the steps you need to follow
a. Find a new company which hires you or ask your current employer to change your residency status to Specified Skilled Worker.
b. The company that will hire you, will proceed with the change of your residency status.
c. During the process, you need to submit your completion certificate of Technical Intern Training Program level 2 and the certification of technical exam number 3 or a recommendation letter from the company for which you worked as Technical Intern Training.
d. During the process, you need to take a medical check (the result will be shared with the Immigration Service Agency of Japan).
The process is different for student visa holders.