I see al lot of NFT projects where you can buy 1 or multiple NFT's for a certain price. Like unsigned_algorithms, CardanoTrees etc. I am curious how these projects are created without the use of smart contracts, is it something that can be done using Blockfrost?
You don't need blockfrost, just learn the cardano-cli and you can make anything. https://developers.cardano.org/docs/native-tokens/minting-nfts/
We can mint NFTs on cardano without smart contracts. There are some projects which help to do it in a browser like https://namiwallet.io/, but you can do it via CLI
The main reason that we can mint NFTs without smart contracts is native tokens on the Cardano blockchain. A good explanation is found here https://www.youtube.com/watch?v=ccLgnM3ErUE at around 3 minutes. There are multiple websites out there now that can take care of the minting process for you. For example: https://metromermaids.com/mint/v0.104/ or https://www.nft-maker.io/ as well as various NFT marketplaces now also offering token minting support.