How do you shorten your resume when you have a lot of things to put in?

In your case, I would focus the attention on the previous jobs, only the last school, and with the side gigs and interesting projects below that shown as one-liners, or in a prose list separated by commas.

If you show too much stuff, or you don't focus their attention on the most important things, you run the risk that they're going to assume you job hop every few months and they're unlikely to want to hire you. But I would expect most reasonable employers to understand the difference between side gigs and interesting projects you've worked on, versus actual jobs.

In my bio for my 30+ year career, I list a bunch of previous employers and companies I've consulted for, and some previous projects I've worked on. It's pretty lengthy. But a bio is not a résumé. You may want to have both, but you need to be careful to separate them by function and focus.

I would do something like this:

List most recent full time position

List second full time position

List freelance/self-employed as a position, and maybe highlight your best accomplishments doing that, and more broadly the general skills of self-management etc, but I wouldn’t rattle off each position.

List educational achievements like just your degree and major.

And then, in terms of your side projects, maybe just highlight the ones that are most relevant to whatever job you’re applying for?

I tailor my own resume to each job I apply for, which means, sometimes I don’t include all my work experience, or all my educational experience. When I used to list all my qualifications, I often got feedback “you’re too overqualified” - so I stick to what is relevant to the role.

Speaking as someone who reads a lot of resumes, my advice is to use space wisely, focus on what exactly you did (too many resumes talk about the team or company) and why you're awesome, and (whenever possible) tailor your resume for each job to prioritize what matches the job description.

Also, while I understand the point of the one-pager, with the massive number of applications I see, a one-pager that's light on relevant info is going to get passed over in favor of a longer resume that highlights germane skills ans experiences.

Your resume is not a biography, it's a sale pitch. Tailor multiple versions of it, one for each specific type of job you are applying for, and drop the things that won't help you close the sale.

Does McDonalds show every product they sell in their ads? Are their ads 45 minutes or 5 full pages long?

Nope, they only show you enough to let you know they have something you want, and they do it within the limits of a regular advertisement.

You need to do that, too. Keep your resume short. It's not a CV or an autobiography. Keep only what you hope will sell you to that particular company, and don't bore them to tears with so much detail they can't get through it.

You're resume is not a biography. It's not a memoir. It exists for one reason, to get interviews. Historic accuracy and completeness is generally out of scope.

To answer your question: Perhaps address skills, tool and on the job experience in one section, and a simple chronological list of engagements in another section.

Editing it for formatting issues can help keep all or most of the meat and fit it on one page.