How can I hang things on my hardiplank siding?

Hardiplank is a cement board. It should be able to withstand the drill. I would call a company that sells it and then you will know for sure. You also need to find a wall stud if you are going to hang a flag as the movement of it, when windy, will put a lot of strain on your siding if you do not anchor it properly.

Also ask what type of screws to use, I’m sure it’ll take a certain kind.

Don't be scared you can fill in the holes

No concern. Use a masonry drill bit since it is cement based. Use Tapcon screws especially for the flag holder. No anchors needed. They are made for concrete, cement, stone.

No need to drill into your siding. Use NoDamageHooks. Very simple, no drilling. https://www.youtube.com/watch?v=6RutcQ1IBhw

https://www.amazon.com/Damage-Hook-Drilling-Installation-Included/dp/B09XLR3KQW/ref=sr_1_3?crid=9MW3CZ7UYGIM&keywords=siding+hooks+for+fiber+cement+board+siding&qid=1652380989&s=hi&sprefix=siding+hooks+for+fiber+cement+board+siding%2Ctools%2C65&sr=1-3