Are SEO tags from API legal?

Question

JavaScript

Interested in how bad it is to make a completely separate application. And if not bad, then how to do it right.
There is a foreign API that returns products with their keywords.

POST /api/v1/product/search 
>>> { 
    term: "Моло"
}
<<< [
    { id: 123, "Молоко" },
    { id: 7, "Молоток" }
]

GET /api/v1/product/123
<<< {
    id: 123,
    title: "Молоко",
    relation: [ 
        {  id: 100, "Сметана" }, 
        {  id: 101, "Йогурт" }
    ],
    price: 26.00
}

And you need to make a separate application on pure JS with direct link routing and SEO optimization.
1. The user enters https://mysite.com/products/milk
2. A request is sent with a search for a product by name
3. We get an ID
4. A request is sent with an ID to get the characteristics of the product and fill in meta tags in the document
How search robots will behave when such a scenario? Will JS be executed on the page? Will they get results? How will they react to dynamic tags? And most importantly, how to implement such a Frontend correctly? So that it works like a SPA, that is, the page is reloaded only when the link is directly followed, and when searching, it works through the HistoryApi