Use Company Losses To Get A Tax Refund

Thanks so much for this info Simon. I'm self-employed and wanted to know: when you're filling out your self-assessment, when do you flag that you want to carry a loss forward?
Do you flag it the same year of the loss? Or do you flag it the following year (when you hopefully make a profit) that you want to use the loss from previous year? Thank you!

If I’m employed and paying taxes through PAYE but also self employed, can the losses from that side business be claimed from taxes paid as employee?

What expenses are allowed to 'create' a loss in a year? For instance, could a company transfer to a director/owner SIPP or SSAS be allowed to boost a loss?

Is this for UK or USA?

for usa or uk?

if my business make 100k and I invest that 100k into a different business does that count as a loss.