You can’t open InDesign files directly in Affinity Publisher. The workaround is to first save the InDesign file as a PDF and then open the PDF in Publisher where it can be worked on. Sometimes this works great, but more often than not, the transfer to Publisher isn’t seamless and needs tweaking or various redos. It works a lot better than one might expect, though.
To answer a previous question, yes, Affinity Publisher can save to PDF and ten or eleven other different formats.
It’s difficult to bite the bullet and leave the Adobe suite. It’s the industry standard and there are lots of things taken for granted with Adobe products until they’re not available and missed. Even so, Affinity’s products are mostly there. I subscribe to both Adobe CC and have purchased copies of all the Affinity apps. If I had to make a choice of my computer being frozen in time with CS3 and switching to a totally modern set of Affinity products, I’d chose Affinity.