Mountain bikes are sturdy and last a long time with minimal maintenance. A lot of the mountain bikes that people use for commuting are 10 years old or more and may be hand-me-downs or bought cheaply 2nd or 3rd hand.
For beginner commuters, my advice is to get something cheap (probably used), so you won't be super heartbroken if the bike gets stolen. For short rides, how well the bike fits you is more important than any other feature. As you get more experienced, you may decide you really want certain features and then you can buy a better bike, and by then you'll be better informed on how to protect your bike against theft.
Alloy and aluminum are used interchangeably. Steel is also an alloy, but for some reason no one ever calls steel bikes "alloy". The frame is only a fraction of the total bike weight. A bike with a 5 pound frame may weigh 20 or 30 or 40 pounds once you add in the components and accessories.