Often, but not always. Awareness-level training is usually fine online. Anything that requires you to demonstrate a physical skill, such as putting on a harness, performing CPR or using an extinguisher, generally is not, because somebody needs to watch you do it. If you are unsure, ask your employer or the body you answer to before you buy.
It varies by country, and sometimes by state or province. Ontario requires a hands-on practical for working at heights. The Australian White Card can be completed fully online in Western Australia and Tasmania, but not in New South Wales or Queensland. Ireland’s Safe Pass is an in-person programme. UK first aid at work requires a practical assessment.
Which country the course is written for, or that it is for use in any country. Where the provider names them, the regulatory body whose rules it follows and the awarding body that accredits it. If a practical is needed, the page says so before you buy, and the certificate is not issued for the online part alone.
Usually they are valid as awareness training and worthless as evidence. If a course does not name an awarding body and tell you which legislation it is written against, assume an inspector will treat it the same way.
One to three years for most workplace safety subjects, depending on the qualification and the country. The refresher interval is stated on every course page, and we email before yours expires.
If it came from us, it has a code and you can check it in about five seconds without an account. If it has no verifiable code at all, that itself tells you something.