- As I read when a transaction is broadcasted, at first it gets confirmed by active nodes and later goes to mempool and miners pick the ones with higher fees. So the question is, when I make a transaction on a website how can I make sure that the website is broadcasting my transaction? Maybe the site is holding it because it has high fee so that he can include it in his own block?
The vast majority of BTC-transacting services and wallets are not miners. They have no incentive to hold your transaction to include in their own blocks. You can also verify that a tx has been broadcast by looking up the txid on an unrelated blockexplorer.
- I noticed that nodes are becoming fewer every day , does this mean when they become less the security of bitcoin or its value will decrease ?
While fewer nodes do have a negative effect on the network, they will not really decrease the security (although if the number of nodes really falls off a cliff it could allow attackers to more easily divide the network and cause a chainsplit; however, I do not have any concrete numbers for this). As long as there is a copy of the blockchain available, and enough nodes to route to each other safely, the network will carry on.
- In order to become a miner, do you have to be a node too?
In the original implementation, all nodes were meant to be miners. Today, since the advent of ASICs, this is impractical. If you are mining as part of a pool, you do not need your own node. If you are running a pool, you do.
- Btc.com has a crazy fast hash rate. Is it even worth to get into mining?
BTC.com is a mining pool, one of many. Their hashrate is comprised of the hashrate of all of their members. If you are able to contribute a high enough hashrate with cheap enough electricity, it can still be profitable to mine.
- Say I want to become a miner with a chance of solving at least one block in a month, approximately how much should I invest? And should I buy cloud space or mine on my own?
It's difficult to answer this authoritatively, as the network difficulty is always changing (every 2016 blocks). Solo mining a block every month, even with ASICs and cheap electricity, is impractical without investing millions in hardware (at which point you might as well just run your own pool).
Most people mine as part of a pool, which tends to find multiple blocks in a day. Each person is then paid a share from each block, proportional to the amount of work they did. This technique can be profitable if you have cheap enough electricity, and the price of BTC doesn't drop too much. However, even this requires upfront investment into one or more ASIC miners, which can cost upwards of $1000 (an S9 costs $1010 at the time of writing).