A web app is an application that is run through the browser, it does not need to be installed on your computer. Web apps are cross-platform, so a mac can run it just the same as a computer with windows, android, etc. As for how to create it, there are many ways, I suggest using something popular, something a lot of developers are using. The most popular framework for front-end right now is React, which employs JavaScript and there are multiple ways to make that front-end communicate with a database. ASP.NET is what is most similar to Access, but its community is highly specialized, it's also a much smaller community.
There are online services that help you create web apps, they're typically limited and opinionated, but by all means, I suggest you don't start without a framework.
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