[Thebroser]

If you pick up an intro college level bio book that might be enough to start out with. My university publishes this so it would be a nice free start -> https://openstax.org/details/books/biology-2e

After that, you could maybe get interested into a anatomy/physiology or systems physiology textbook.

For more specialized content maybe NCBI StatPearls -> https://www.ncbi.nlm.nih.gov/books/NBK430685/

Note that due to the ever changing landscape of biology, only way to truly ever "be an expert" is to actively participate in the life sciences and keep up with contemporary research articles.