I would hire an SEO consultant to check your exact situation and give their advice, but top of my head here are some steps I would do:
Contact your domain registrar to see if you can forward all requests to the old domain to the new domain (ideally matching the sub folders / exact url ending, but if not that then forward to the new domain’s home page).
Identify all backlinks using Google Search Console, then update those backlinks (or on 3rd party sites, request to update the link) so that the link is fresh, pointing to your new site.
This is a good time to upgrade your 404 error page, for example add some messaging and/or carousels with popular products.