there are a number of existing threads that might help. If you look to the right under "Releated Community content" you will find some or you can also search using the Search bar at the top of the page.
Here is one that has a number of good references - zoho-to-jira-intergration
here is an article on CSV import - creating-issues-using-the-csv-importer
here is a post that discusses importing attachments - Does-importing-issues-from-another-system-with-attachment
It depends on what are your projects based on. If they are based on a relational database, you can try out Skyvia. It allows you to import necessary Zoho data to Jira automatically (https://skyvia.com/data-integration/integrate-zoho-crm-jira).
Skyvia is Jira's partner. I found it on the Atlassian marketplace
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