ManyToMany

Account.java

// ...
@Entity
public class Account {
    // ...
    @OneToMany(mappedBy = "account")
    private List<AccountTag> accountTags = new ArrayList<>();
}

AccountService.java

// ...
@RequiredArgsConstructor
@Transactional
@Service
public class AccountService {
  
    private final AccountRepository accountRepository;
    private final TagRepository tagRepository;
    private final AccountTagRepository accountTagRepository;
  
    public void addTag(Account account, String tagTitle) {
        Tag tag = tagRepository.findByTitle(tagTitle)
                .orElse(tagRepository.save(Tag.builder().title(tagTitle).build()));
        Account byId = accountRepository.findById(account.getId())
                .orElseThrow(() -> new UsernameNotFoundException(account.getNickname()));

        accountTagRepository.save(AccountTag.builder()
                .account(byId).tag(tag).build());
    }
}

AccountTag.java

// ...
@Entity
public class AccountTag {

    @Id @GeneratedValue
    private Long id;

    @ManyToOne
    @JoinColumn(name = "account_id")
    private Account account;

    @ManyToOne
    @JoinColumn(name = "tag_id")
    private Tag tag;

    @Builder
    public AccountTag(Account account, Tag tag){
       this.account = account;
       this.tag = tag;
       addTag();
    }

    private void addTag(){
        account.getAccountTags().add(this);
        tag.getAccountTags().add(this);
    }
}

Tag.java

// ...
@Entity
public class Tag {
    // ...
    @OneToMany(mappedBy = "tag")
    private List<AccountTag> accountTags = new ArrayList<>();
}