This already exists
https://shopify.dev/themes/product-merchandising/recommendations
https://shopify.dev/api/ajax/reference/product-recommendations
That’s a broad overly ambiguous target.
Look into personalization engines and discussions, or just search the app forums here for “product recommendation” etc though you’ll have to filter through a bunch of junk because of the word “recommend”.
A goto for beginners would have been google-optimize product personalization but gooptimize got sunset in september.
DIY’ing this would require being much more specific and good understanding of browsers, javascript and shopify and/or it’s apis.
Good Hunting.