How to focus on effectively applying for jobs with reputable employers?

So should I just keep watch of their job posting and apply if it's a good match?

Yes

Should I try to figure out specific information, such as their tech stack, so I can practice it before a job opening comes up?

Yes, but no need to Erotemata on this. Either you have the tech skills, or you don't: cramming for a few days won't make much of difference. Related skills are often good enough

... "informational interviews" ... How exactly is this supposed to work?

That happens rarely and is typically constrained to job fairs, industry events, etc. or other larger gatherings where are a lot of relevant people are on one spot. I have never seen "informational interviews" in-house.

If you already know which company you want to work for, and they have an office in your area, how would this change your job search/application approach compared to just applying to random companies?

1. Monitor their websites(s) for job posting
2. Gather as much about publicly available information the company as you can
3. Try to network: attend relevant professional society meetings, related clubs or meet-ups, and just keep your eyes and ears open for people that work there and could help you make contact. Keep it professional, don't stalk.
4. Once you have a solid body of research you can pro-actively reach out to the recruiting department with a message that clearly shows that you have done your homework. You need to impress the recruiters with the quality of your application and underlying research.

IMO quality beats quantity anytime. If you see a promising opening (local or not), take the time to research the company and write a thoughtful and customized cover letter that shows that you care. High volume scatter shot applications don't help anyone.

I can't say enough about #3 from @hilmar:

Try to network: attend relevant professional society meetings, related clubs or meet-ups, and just keep your eyes and ears open for people that work there and could help you make contact. Keep it professional, don't stalk.

Right now, any real and reputable company is getting absolutely DELUGED with fake resumes and also real resumes. For years and years, big name companies would get so many resumes that recruiters were making hard brutal choices about what resumes they would even read. And there were things like the code games piloted by FANG companies to get a read on someone who can code before they even see a resume.

But in the last 1-2 years, AI has made this so much worse. Added to candidates of the past (people who where maybe overly hopeful about how well their resumes matched the JD), we now have submitters who are actually bent on fraudent interviewing. Companies simply can't keep up, and are working on more layers of pre-human filtering, because they simply don't have the money for hiring the staff. It's a war of the bots.

But what still works, and still works WELL, is making a personal connection. Personal referrals get through a MUCH MUCH higher percentage of the time.

Things I'd do to get my resume actually seen by a company I want:

1 - look at your linked in and see if you have connections to that company. The best referral is someone who's had a strong positive working connection with you.

2 - don't stop at those people. People who vaguely know you may think much better of you than you realize. We rarely express appreciation for each other, and reaching out some-guy-you-met-once from some-random-thing-you-did may yet get you to someone who is way more impressed by you than you realize. If you see these people have a connection to the company, it's a good time to reach out and say hi, and ask for a favor - an intro. Meet up, IRL or online. Send 'em a starbucks gift card if that makes you feel better. Just reach out.

3 - don't stop there - look at second degree connections to recruiters or hiring managers in your field. Find someone from (1) or (2) that you know, who could do a friendly intro to that second degree connection. When you get to the actual person in the company, ask smart questions, offer some stuff about yourself, but make your interest and excitement in the company your focus. At the end of the contact, see if they would refer you.

4 - Go to anything that gets you meeting those people from (3) - meetups at the campus, job fairs, any kind of meet and greet - it's not too stalker-y to scan the public feeds of hiring managers in companies you are interested in and seeing where they are going to network and socialize.

When I was on my last serious hunt, it was so easy to feel like THEY were doing ME a favor - I needed a job so much, and they were so hard to come by. But realize that by showing yourself to be a real, actual human, with the real actual skills -- you actually saved these people from having to talk to a hundred AI fakers instead. This is a mutually beneficial interaction. Not a favor.