Previously I have worked for supermarkets and in Academia, both of which advertised their recognised unions during the onboarding process.
Although I recognise I'm being well compensated at FAANG I would like to join a union for the other benefits they offer, such as legal support and advice.
Research their public profiles: my impression is that some are very much in the “employee advocacy” space (providing enterprise bargaining where appropriate), whereas emerging tech unions in the US (particularly those aligned with specific FAANGs) appear to be more about pushing a particular political line than worker representation and support.
Obviously you’re your own person, I recognise that we should unionise, but selfishly I would be concerned with potential blowback; maybe you don’t pass probation somehow.
If you’re intent on doing this, I heard decent things about prospect: https://prospect.org.uk/tech-workers/
In Sweden it would be unionen, but unions are usually country dependent (for obvious reasons)
You might also put the 15-20 quid a month that an union membership costs into a fund to cover any lawyer consultations (which, let's be honest, are rare).
Lastly, note that as part of redundancy settlement agreements the employer will often pay for your independent legal advice.
The time it takes will vary depending on the rate of change of the industry, but they are doomed.
In tech, things change quickly.