Will your phone take a Jelly?
Two things have to be true: your phone supports eSIM, and it is carrier unlocked. Check the first one here.
Pick your phone above
Check your phone directly: on iPhone look for Settings, Cellular, Add eSIM. On Android look for Network, SIMs, Add eSIM. If that option exists, you are good.
How do I check without the tool?
iPhone
Open Settings, then Cellular. If you see Add eSIM or Add Data Plan, your phone supports eSIM.
Android
Open Settings, then Network and internet, then SIMs. If you see Add eSIM or Download a SIM instead, you are supported.
What is a carrier lock and do I have one?
A locked phone can support eSIM and still refuse a third-party plan. Phones bought outright are usually unlocked. Phones bought on a carrier contract often are not until the contract is paid off. Your carrier can confirm and unlock it, usually for free.