If I setup a website which advertises companies, how is the income from those advertisements taxed?

Or if I decided to charge a subscription fee for the same website, how is subscription income taxed?

I'm mainly curious if they are subject to self-employment tax or not.

If I setup a website which advertises companies, how is the income from those advertisements taxed?========>> People who work for themselves instead of working through an employer receive compensation income based on the fees charged to their clients or customers. your online biz income is your self employment income as you are a self employer filing Sch C/SE , not an EE receiving an W2;

Or if I decided to charge a subscription fee for the same website, how is subscription income taxed?
========>>it is also your self employer income, so,you need to report it on Sch C aslongas the amount on Sch C line 29/ 31 is $400 or exceeds $400 then you also need to pay self employment tax. You can deduct 50% of your seca tax on line 27 form 1040.
If you are filing as a sole proprietor and/or a self-employed individual, you generally have to make estimated tax payments if you expect to owe tax of $1,000 or more when you file your return
.however,
You do not have to pay estimated tax for the current year if you had no tax liability for the prior year; you were a U.S. citizen or resident for the whole year; your prior tax year covered a 12 month period

I'm mainly curious if they are subject to self-employment tax or not.=======>>>>>>>>>>as mentioned above; yes.
Self-employment tax is a tax consisting of Social Security and Medicare taxes primarily for individuals who work for themselves. It is similar to the Social Security and Medicare taxes withheld from the pay of most wage earners.