Where is the springboot key file stored?

I am an Android engineer trying server deployment for my overseas applications, using springboot and vultr as deployment. Third-party login uses firebase, which requires spring to hold a secret key json, so which directory should be stored in order to be relatively secure?
I closed the password login for port 22 on the server, ensured the login with ssh secret key, applied for a free https certificate and used it, what other common security issues should I pay attention to?
Thank you for your advice! If it is best to instruct me about the key words and related links, my current server level can only use springboot to write some simple business related to mongodb. -sharp-sharp-sharp problem description

Jun.10,2022

if it is particularly necessary, it is recommended that you keep the ciphertext in db. Briefly explain the reason for this. If there is no security breach in the application itself, the path from the longest hack to the key:

  1. Server, account, password
  2. db address, account number, password
  3. decryption of ciphertext
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b369d8-2c011.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b369d8-2c011.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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