collect a load of transactions (somehow): by this you mean the mempool. the implementation is client dependent, it uses the p2p network to receive new txs, thats what you need to look at to know how to "get txs from the network". some implementations have better documentation. if you need to look at code i would recommend the go implementation https://github.com/btcsuite just because it looks cleaner to me.

validate them: your best resource for this is code. your client needs to keep old consensus code around and use it to verify block under the older rules (your client will do a full sync at some point)

rules for formatting a transactions and blocks (and a byte by byte explanation) can be found on https://learnmeabitcoin.com/ as mentioned (probably will be up soon)

keep changing a certain bit of the block until SHA256(f(SHA256(block))) is low enough (with some f: digest → bitstring): yes. you set the block nonce to mine. finding a block means that the hash256() operation (sha256(sha256(block))) returns an hash with at least D (either left or right most, can't recall right now) bits set to 0, where D is the current difficult, which is adjusted by the network to make the 10min/block average time