Annual allowance + carry forward: can someone sanity-check my understanding?

I think I understand the pension annual allowance, but every time I read about it I lose confidence. My current understanding (please correct me):

• “Annual allowance” is basically the max you can put into pensions in a tax year before extra tax applies (including employer contributions). Source
• You may be able to use unused allowance from previous years via carry forward (but the rules feel… fiddly). Where I’m stuck:
• When people say “you can contribute up to £X”, are they talking gross or net (after tax relief)?
• If I do salary sacrifice, is that counted the same way as personal contributions for allowance purposes?
• What’s the cleanest way to check if I’m at risk of breaching it before I accidentally do? If anyone has a simple checklist they use each tax year, I’d really appreciate it.