If you have WordPress installed in MAMP on your local computer then you will only be able to access your local WordPress website when MAMP is running. MAMP is the Apache webserver, database server and PHP running locally on your computer – turn that off and your WordPress website can no longer run.
If you are brand new to local development it’s important to understand that running a WordPress website in MAMP on your local computer does not make that WordPress website publically available to anyone else on the internet – it can only be accessed on your local network.
MAMP and other tools like Local by Flywheel and Desktop Server allows us to do local development of our websites without having to be connected to the internet. Once the website is ready to launch we then have to export it the website files and database and import them into a production web hosting environment on the internet where it is publically accessible to everyone.
So if you’re wanting to use MAMP for local development that only you can see, that’s great. If you’re wanting to build a website in MAMP that others can visit then you may have misunderstood what MAMP is used for and will need to find traditional website hosting for your self-hosted WordPress site or alternatively use a WordPress.com site where the technical installation side is all taken care of for you.