Preparing for a driving theory or permit test is one of the few situations where the free material is genuinely the best material, because the test is written from a specific official handbook and nothing else.
Start with the official handbook
Every licensing authority publishes a driver handbook, free, and every question on the test comes from it. Downloading it and reading it properly is the single most effective thing you can do.
Many authorities also publish sample questions or an official practice test on their own site. These are the closest thing to the real examination that exists.
Make sure you have the current edition and the one for your jurisdiction, since rules differ meaningfully between them.
What paid apps actually offer
Most practice test apps repackage the official material in a more convenient format. Some add useful features: tracking which topics you get wrong, spaced repetition, and question banks larger than the official sample.
That convenience is worth something to some people. It is not worth much if you have read the handbook, because the handbook is the source.
Where free apps make money
Free practice apps typically carry advertising, sell an ad-free upgrade, or route you to paid services such as insurance quotes or driving schools. That is fine, but be aware you are the product, and be careful about what personal information you enter.
Avoid anything asking for payment details for a free test, and be sceptical of sites presenting themselves as official when they are not. Official sites use government domains.
What the test actually covers
Typically road signs, right of way rules, speed limits, alcohol and drug regulations, parking rules, safe following distances, and procedures for specific situations such as school buses, emergency vehicles and railway crossings.
The areas people most commonly fail on are right of way at intersections, sign recognition, and the specific numeric rules such as distances and blood alcohol limits. These reward memorisation, so target them.
How to prepare efficiently
Read the handbook once fully, then take a practice test to find your weak areas. Reread only those sections, then retake. Repeat until you are consistently well above the pass mark, since test-day nerves cost a few points.
Most people who fail did not read the handbook and relied entirely on practice questions, which teaches you the answers to those specific questions rather than the underlying rules.
On the day
Check what identification and documentation the office requires before going, since being turned away for missing paperwork is the most common wasted trip. Many authorities now allow booking an appointment online, which saves considerable waiting.
Ask what happens if you fail, since rules on retesting and waiting periods vary.