How to indent a block of text on a MacBook Pro? In Mathematica, if I put the marker right next to a line of text and press Tab, the line indents, but if I mark a block of text and press Tab the whole text is replaces with an indent.

I've tried different combinations like shift + Tab, option + Tab, none of them work.
It's up to the app to support this. Pressing tab inserts a tab character, so if you have text selected, it will be replaced by the tab. The app must intercept the tab key and perform a special action of adding the tab to the start of each selected line.

You can quickly add a tab to the start of a line by moving the cursor to the start using ⌘← then pressing ⇥ to add the tab.

You can use a Keyboard Maestro macro to add block indent functionality to apps: