The education non-profit Khan Academy offers an official Digital SAT Prep course that is partnered with the College Board. This course helps you focus on practicing the exact skills needed to succeed on the SAT forfree.
Started by the CEO of Khan Academy Sal Khan, Schoolhouse is a free tutoring platform that gives students access to SAT tutors. These tutors explain step-by-step how to answer SAT question that you may get on your exam. Also, Schoolhouse hosts SAT bootcamps throughout the year.
Bluebook is the official Digital SAT practice test application that gives you access to 10 free practice test. This can help you practice real SAT questions and time management.
Youtube offers a plethora of SAT/ACT prep that is both helpful and free of charge. Many SAT/ACT test experts post videos about how to approach questions on the SAT and ACT, and sometimes do walkthroughs of practice tests. Some good channels to check out are 5 Academy (ACT) , Scalar Learning (ACT), Prep Pros (SAT/ACT)
The Library
Many local libraries have access to SAT/ACT prep books that are free to borrow with a library card (usually free). I suggest that you borrow an SAT/ACT prep book that is fairly new ( at least a 2020 edition) for more effective studying. Additionally, you should highly consider borrowing books to read from the library. Studies have shown that students who score higher on the SAT/ACT read frequently. Reading often can help you improve reading comprehension skills and learn vocabulary.