Celebrities are not influencers. This is an important distinction.
Celebrities are public figures who established their popularity offline, in most cases for certain talents in singing, sports, acting etc.
Influencers are public figures who established their popularity online, usually for their expertise in a specific area of interest and shared via content they created themselves.
When it comes to advertising, celebrities have broader appeal and are therefore most ideal for generating mass awareness. Influencers have a relatively more niche appeal. What makes influencers so powerful is that they are often trusted experts and have closer relationships with their followers.
There are two very different forms of influence taking place when a celebrity singer recommends a brand of olive oil compared to when a foodie influencer recommends that same brand of olive oil.
Budget is the number one factor that determines the use of celebrities or influencers. Additionally, the platform can play a large factor. For example, there are very few celebrities on Xiaohongshu, so influencers are essentially the only option for brands that want to advertise on the platform.