DNS zone changes have to be made on the platform where your domain is currently pointing.
VPS plan + domain points to Hostinger
Let's say your domain points to VPS server's IP via A record, though your Name Servers point to Hostinger. You can find out the name servers of Hostinger HERE.
That means that all DNS changes have to be made on Domains section under DNS zone.
Shared plan + domain points to Hostinger
If your domain is pointed to Hostinger name servers:
and is hosted on Shared or Cloud hosting plan, you should add or edit any records here:
First click on the Hosting menu.
Click on Manage next to your hosting account.
Cloudflare (or other platform) + domain doesn't point to Hostinger
If your domain is pointed to any other host or even Cloudflare via Name Servers. That means that all of the records (TXT, MX and so on) have to be added there.
Here is how you can edit DNS zone on Cloudflare: