Dear, it's not a big deal like asking questions being a newbie there is no shame. I even appreciate you asking questions, Here are a lot of your senior members who will advise you the best suggestions that I think you may miss while doing your own research but yes once you have read many suggestions after that it is very important to do your own research which is the most important thing to make yourself more secure in crypto space.

Besides all that the answer to your first question is that you can buy coins from exchanges and send them to your personal wallet using your wallet address or if you want to buy any Coin through the wallet you can change the wallet network to the token that you want to buy, I mean to change the wallet network to the token supportive network. secondly, Yes you have to transfer your token to the supportive exchange where you can sell your token and convert them into USDT, and then you can sell USDT which will further convert it into your country currency. Thirdly, There are a lot of projects or tokens that are not supportive to your single wallet even though there may be no network in your wallet for that so for that you have to download some other wallet that supports specific tokens you want you to buy but keep in mind that you can import you personal wallet into the new downloading wallet by using your secret key or seed phrase. DYOR. Many Thanks!