How difficult is it for a High School Senior to get a summer job in the tech industry?

I would suggest offering to work for free. This can gain you invaluable experience and be a good stepping stone to the job you want.

It's pretty competitive. I would seek out the less well-known firms and quickly.

It is a competitive world. However, nothing is impossible. It just takes time and dedication to find the right people/right jobs.

My recommendation will be reaching out to people who currently work in the tech industry (friends, parents of friends, etc) and do a summer internship for you to observe how the tech industry is like. The term "Tech Industry" is so broad and has multiple aspects to this. Try reading up some articles about the latest trends in technologies (Business Insider, TechCrunch are good magazines) so you can have a meaningful conversation with the companies trying to hire.

When you say "tech industry" I guess you mean "software technology."

If you know somebody in a company you're interested in, ask for a summer job. Ask them if they know anyone else who has a summer job to offer.

Go to meetups, etc. Introduce yourself. Get yourself some cheesy business cards with your name, email, phone number, and "looking for a summer job" on them. You can get those for free / cheap online or make them yourself with card stock, a printer, and a paper cutter.

Look on the hiring boards (craigslist, monster, dice, etc) for companies in your area. Doing this, you're not looking for job openings, you're looking for companies. Write them emails saying what you want, what you can offer, and when you're available -- start and end dates. Most of them won't respond. Don't worry about it.

Keep in mind that a summer job with a fixed end date is an easier hire to make than a permanent job for your employer. If you work out great, that's terrific. If not, you disappear at the end of the summer anyway, so you save your manager the hassle of sacking you. I'm not saying you won't work out. I am saying summer hires are less risky than full-time hires for the hiring manager: that gives you an advantage.

You'll probably do testing work or similar low-risk work at least in your first few weeks.

Go for it. I got my start with a summer job. It's a good way to go.

Tech industry is a very broad field, it depends which aspects of it interest you the most. I would do research on those first, and if you are not sure, just be open minded and try anything that comes your way. By trying and experiencing various jobs, you will learn faster not only what is out there, but also what interests you and what you may be good at. Remember, when you are as young as yourself, often you don't know what you don't know.
Sometimes is just a matter of being at the right place at the right time...but also, being ready when the right place and time comes along... And the only way to find out how difficult it is or not, is to try... the more you invest in it, the easier it becomes. And only the persistent succeed, so do not overthink it, just go for it and see where you land. Good luck!