I know that this is not a big concern but I've always seen these folks spell Ethereum as Etherium, I wonder where you're getting that when you were able to buy it from any of those exchanges and have it abbreviated only as ETH but if it's on metamask, it's spelled correctly on the mainchain. Anyway.
1. Do i do the same and open various wallets and buy the coins that way or do I have to use an exchange like Coinbase or Binance to buy the coins?
Download a wallet that supports a lot of cryptos like Atomic, TrustWallet, Exodus, Coin Space or even purchase hardware wallets.
2. If/when I want to sell the coins do i withdraw via the wallets or do i go onto an exchange and sell? Then transfer the money to my bank account.
There are wallets that have internal exchanges but fees there are quite high because they get commission from every sale or trade.
3. Is there a secure wallet I can get with can hold all the coins in 1 place or do i have to open multiple wallets?
A hardware wallet is your best tip.