Search is not about keywords and has not been since 1997 when Google came onto the scene as a semantics based search engine. Granted, when Google was new, implementation of semantics was far less sophisticated than it is now. Today, the implementation of semantics is staggering and far more complex than ever imagined or even realized by SEOs to this day. The notion of keywords is derived from the highlighting of search terms found in search results after very many algorithms have been applied. This effect is incidental and not actual. At no point does Google make term matches. This is especially true with the introduction of RankBrain which has always existed to some lesser level. So please, ignore the SEOs that take about keywords and throw this notion out of your mind forever.
What works are complete and descriptive sentences and not keywords.
To focus on keywords is to miss the point. What happens when a site owner focuses on keywords is to skew the semantic results narrowing how their site is actually found to a far narrower search market.
Be that as it may.
The difference between Best baby stroller and Best car sit baby stroller and an understanding of semantics makes it clear that only one is preferred. But let me make a correction to your phrase Best car sit baby stroller first. It is more correct to use Best car seat baby stroller. Primarily, sit is a verb and seat is a noun and the association between car, sit, and baby will be mistaken for the action of a baby sitting in a car rather than a description of your product.
Let me go further. Best car seat baby stroller has Best baby stroller within it. Semantically speaking the first is stronger the the second.
When trying to rank pages for search, it is always best to be natural, however, it is also best to also be descriptive and provide as many clues to your product as is reasonably possible. This can be spread through out your content.
I use the question What about...?. For example, what about car seat, baby stroller? What is important for you to say about your product? What are people looking for? What concerns do people have? What is popular? What is often missed?
Search is not about search trends. These mostly only apply to trend based sites such as celebrity news sites. Instead, you want to tap into not what search engines say are trending, but what the user is looking for, is concerned about, does not know, needs to know, what others say, etc. Following keywords and trend searches will only drive you away from the so-called long-tail search potential.
For example, it may not advantage you to use the term best. Who is searching for best car seat? Is it that anyone who is searching for the best car seat is using the term quality, safest, etc.? If you are writing content such as The DXP brand child car seat meets and exceeds AAA safety standards making it the safest child car seat available today., then you will likely see better search results.