Usually both, this is the fundamental lack of understanding most people (even web, SEO and ecommerce people) have with reviews in general.
This should form part of your marketing strategy; e.g. what are you generating reviews for and where will customers see them?
Pretty much all reviews can be shown on your website, regardless of review source. There are tools / plugins / JavaScript widgets for most review platforms to show reviews on your website. We've built a tool that can display reviews from well over a 100 review sources on WordPress websites.
You are able to show reviews in Google/Bing Ads and organic search too, which often improves clicks and organic ranking. This is where understanding how reviews, search results and advertising work determines your marketing/review plan.
You may well need a mixture of brand (Google Business Profile) and product/service reviews too, this means using more than one platform usually, as people look on Google Maps/Business listings and at organic and/or paid results.
This can be quite complex, it's too long for a forum post, but we've explained it here:
https://insiteweb.co.uk/review-generation-strategies/. Feel free to get in touch if you need any help, as it's a service we provide, and we can usually get you better costs than the big players for generating reviews at scale.