What reading apps would be best for me?

Hello, I want to get a job but want to make sure I know what I’m getting myself into. I E-read mainly on my iPhone on the books app and wanted to know what would be a good way to seamlessly carry over reading between both my phone and the kobo. My question here mainly being is there an app that I can download on the kobo and my phone where I can read the same book at home/outings but then pick up where I left off on my phone when I’m at work when I have unexpected down time since I won’t be able to bring in an E-Reader with me for a “maybe” on my possibility to read.

Any suggestions are appreciated I just mainly want to avoid having to buy a book twice digitally if I can. Worst case I plan on having two different series one on the e reader and the other on my phone .