jbstore if you can share the information weather you are using the main domain or an addon domain we can pinpoint you to the right guide to change the DocumentRoot
so /public
is hidden from your URL. Alternatively as mentioned by RelicSystem you can also try contacting your hosting provider to assist.
About installing the extensions: You should make yourself familiar with basic SSH knowledge to navigate composer. Installing, updating and removing extensions will always involve using composer commands in SSH. If you aren't familiar with those you'd always be depended on external help.
If you don't want to get familiar with using composer / SSH then you might also want to consider a managed hosting for your community - which for example can be https://www.freeflarum.com or https://blomstra.net