Publishers tend to look to other authors for book reviews, because they can add statements from these authors that give the promoted book weight.
You can look at blogs, Amazon and Goodreads for samples of reviews.
A lot of book reviewers start their own blogs/websites and authors seek them out for reviews once they have achieved a following. A book reviewer with a large audience is classed as an influencer and can be heavily in demand.
Reedsy, which is highly-regarded in the publishing community, has a list of bloggers who do reviews and you could look at their sites for inspiration:
Best Book Review Blogs of 2019
The secret is to get started. Review on Amazon and Goodreads for practice, start your own review blog, and gather your audience.
One tip though: always try to see the good and the bad in things you review. Don't look for fault or confuse criticism with critique. Writing a book, even a bad one, is an extremely challenging and difficult task. And writers see their creations like their babies, thrust out into the world. So, think carefully before you bludgeon it with a brick.
Writers remember reviewers who are unnecessarily harsh and steer clear of them when looking for reviewers.
p.s. I have a book coming out later this year, you can review that if you like!