Is it possible to get side loaded books available on Phone and PC Kindle apps?

For Kindle PC, you need to make sure this book is mobi format, then directly open this book with Kindle PC app.

For Kindle iPhone app, you will need the Kindle cloud service. In another word, you need to transfer the book to your Kindle cloud at first, then sync it to your iPhone. Here is the related guide: https://www.ereader-palace.com/upload-epubs-kindle-reader-kindle-app/