The IRS has what they call Free File, which, true to its name, lets you file your federal taxes online for free, regardless of how many forms/schedules you need.
What confuses people sometimes is the mention of the income limit on the page. If your income is below $66,000 (as of 2019), then you can use the Free File Software, which like other tax software will ask you questions and guide you through the process.
On the other hand, if your income is above $66,000, you only have access to Free File Fillable Forms, which is literally just you filling out the forms online. It does do the basic math for you (e.g. add lines 1 through 3 and write the sum on line 4), but other than that, provides no guidance whatsoever.
What About State Taxes?
Your state may have a similar program for everyone to use for free.
If your state does not, beware: while many of the popular tax software programs such as TaxAct and H&R Block do allow you to file state taxes for free online if you make below a certain income (and sometimes, if your tax return is sufficiently simple - e.g. not requiring Schedule D, E, etc.), as far as I know all of them will require you to use their program for filing federal taxes as well.
Hence, if you are in a state where the state government doesn't run their own free online tax filing program, forcing you to use the tax software of a third party company, you will most likely not be able to use the IRS's Free File for your federal tax return.