This is marketing, people don't utilize old school marketing anymore, with the growth of social media, now people and companies focus more on influencers, and affiliates they want real people posting about their products and that person gets a kick back, now sometimes is a small comission based on purchases off your affliate links, or itsbased off your following base, etc sometimes you just get free product
sponsorship is just another marketing term,
Ideally you and the company will want to come to an agreement on what you feel is fair to you and them, some companies will take advantage and some people will try to take advantage of small businesses by asking for free products in exchange for review ets,
Its your choice if you want too,you will have to read their agreements, you are essentially entering into a contractual agreement most times with a company,
Lets say Sara,is now an affiliate for oldnavy she is required to post x amount of times per week with her affiliate link and linking all of old navys products, she is required by law to post a paid partnership label on instagram including adding to her post this is an Ad that is required by law,
alot of companies are able to get around some laws due to their location, but that doesn't mean you arent breaking the law by not posting its an ad etc,
I work as a small business owner, i don't do affiliates or influencers, its not my marketing jam but as you all know i also do the entire advertising on here as well.
Basically you will want to review and read ALL your terms and conditions sometimes companies make it very simple and other times not so much