OP: you ask the big questions that lead to a deep rabbit hole of digging into research, so lets take things easy
Being new to BTC I would like to know where the BTC are held that are for sale?
firstly.
a BTC is a lump of satoshis(sats). every value measure of the bitcoin blockchain, measure its value and transfers its value in units of satoshi's(sats)..
there is no BTC in the digital or real world.. a BTC is just a name of a measure of allotment of satoshis. 100,000,000sats infact =1btc
from the first genesis block to now all block rewards and transactions are measured in sats
BTC only exists as a device display measure or in a simplification of technicalities in a conversation, (calculating a lump of 100,000,000sats into 1btc, people jsut say 1btc for short)
its simply easier to use lump numbers to say there will be a total of 21m btc instead of saying there will be 2,099,999,997,690,000sats
much like in the real world there is probably no actual real world single 1tonne brick of gold. however total gold supply is talked about in terms of gold tonnes.. the reality is we actually have ounce coins and kg ingots.. but no real "gold tonne" so think of BTC as "gold tonne" where sats are like real ounces of actual gold
bitcoin value (sats) are stored on bitcoins blockchain. its a ledger of all value(sat) created and subsequent transfers to know where the current owner of a set of sats is. there is no single centralised ledger blockchain. instead there is a beeutiful mechanism that allows thousands of pieces of software to compare their ledger of bitcoin to other ledger via the latest blockhash.. this allows everyone to have a copy that sync with others so everyone shares the same data. and if one person edits their copy. only the editor is seen as out of sync and thus ignored. and so no single person can change the blockchain for all parties
bitcoin value is transfered from and to owners/custodians via transactions logged on the blockchain, of peoples bitcoin addresses sending or reciving amounts of sats.. which these logged addresses have paired private keys to prove ownership of the addresses in question and authorise transfer of funds from their address to another address without revealing the private key on the blockchain
you can directly transfer bitcoin value from your address(after you funded it) to someone else via non public communications arrangements. to then create a transaction to then broadcast the transaction so it appear and is confirmed and settled on the blockchain.. for all to see
however most people prefer to transfer sats (lumps: btc) into the address of a custodial or exchange service whereby the service get sats logged to their address which they have the private key for, thus making them technically the owners of the sats
but they act as custodian and manager 'entrusted' on your behalf, for you to request the sale or ownership transfer of the value via there software/platform/app
which is where they act as exchange
Other than private wallets, is there a pool of BTC, and how many are there of the 21000000 total available?
currently as of todays date ~1996783750000000sats (rounded to nearest half btc of time of posting..)have been created (19,967,837.5 btc(19.967m btc))
these are not in any single pool/central lump all combined.. they are in lots of addresses some holding just 100sat some holding (btc measure) a couple million btc
currently services like coinbase holds entrust a few million btc measure of sats, other services like binance have about (easy measure) 520k btc
I am looking to buy more BTC ETF before I go deeper into buying the actual BTC wallet.
a bitcoin ETF is not an ownership claim of actual bitcoin value. what you will find is a couple years ago your ETF broker/portfolio manager acted as sponsor to a ETF like blackrock whereby the sponsor bought bitcoin value. gave it to blockrock entrust, where blackrock put it into coinbase as custodian entrust. so that blackrock then can issue shares of a trust fund valued at the bitcoin valuation cap of that lump it has. to then allow the sponsor(your broker) to sell you ETF shares
these shares are not ownership claims of bitcoin value(sats). they are just shares that represent a split up of a trustfunds valuation which pegs(follows) the bitcoin market price.. inshort you own no sats when having ETF shares
it is better to actually own bitcoin sats on your own address which you have a private key for, than to have shares that can be diluted or the sponsor/broker go into liquidation/bankruptcy