Im trying to elevate someones permissions so that they can edit and add users etc across all site collections in our farm. I added the user into the farm admins group but it doesnt look like their permissions have been elevated. What exactley does adding a user to the farm admins do and how can i grant full controll to everything in a farm?
Being a Farm Admin lets them utterly obliterate your entire farm with a few miss-clicks so I'm pretty sure that you will want to remove them from that group quickly.
What it sounds like you are looking for might be a Web Application Policy and that can be configured in Central Administration - Security. Simply add them to the Web Application desired and with Full Control permissions. Be forewarned though that permissions set this way unconditionally override any permissions defined in the sites themselves, even item-level permissions.
If that is too much power (and it really is), then consider using a PowerShell script and adding them as either Site Collection Administrators or (preferably) to the default Site Owners group for each site collection. The specifics of the script will vary by farm configuration but this is a good starting point.
In general, you will want to give users the least permissions possible to do the job so I would strongly recommend adding them to the Site Owners collection.
If this is something of a common request then consider creating an Active Directory group instead and then using the aforementioned script to add the AD Group to the Owners SP Group instead of the User ID. Then as users come and go, you simply add/remove them from the AD group instead of fiddling with SharePoint permissions each time.