You may have heard the saying "You don't find a Technical Co-founder, you earn them"

But what does that mean exactly?

It means you think of them like you would an investor. So you should...

Build your startup as far as you can without them. Pitch decks, designs, partners, investors, advisors, prototypes using freelancers, in-market product using yet more freelancers etc. Make your business worth investing in.
Have a process. Execute it well. That includes...

a) Find the senior engineers at the companies you admire who seem to have a background in the kind of work you're doing
b) Reach out to them (LinkedIn is great for this) with a message that is highly tailored and extremely leaned in (remember that they are being bombarded by recruiters all day long - make sure to point out you're the CEO). Don't be coy, don't be dry. Explain why you think they might change the trajectory of your company and why its a MASSIVE opportunity for them. Ask for just 15 minutes of their time to explain the rest on a call
c) Offer them a meaningful piece of the business with a 4-year vesting schedule and a 1-year cliff.