How to fix a WP site built by someone else

I have been given a 90% complete WordPress business web site to finish. I’m competent in PHP, HTML, CSS, and so on, but I’m a WordPress newbie (so please forgive any terminology mistakes I may make). All the web sites that I’ve been involved with before have been hand-coded.

I’ve already managed to find and fix quite a number of issues with the site, but I’m having troubles with some things that just don’t make sense to me. The site is apparently based on the 2015 theme, but it seems that there are a lot of pages within the site that have their own templates, and there are about a dozen plugins enabled.

Because I didn’t design the site, I feel like I’m having to look at it backwards. There are pages on the site where I can see the end result, but I can’t work out how that result is being created – where the text and images are coming from, why they appear where they do, and other similar questions.

Are there any plugins or tools that will help me understand how the site works? For example, how can I tell which template is used to display a particular page? How can I tell where the content in a page comes from?

Any suggestions you have would be gratefully received!

Regards,
Peter