like for example: react, C# WebM, WebP, av1, and many more
some parts that big tech made, are really hard to replace, due to them, being complex or fundamental to a speific system
how would we replace them (i hope there are existing replacements for the examples i talked about)?
This is pretty off-base for what OP is asking about.
React and C# are open source.
WebM, WebP, and AV1 are open standards that have mature and well developed open source implementations.
Literally every item OP asked about is already OSS. OP just doesn’t like that they originated at Meta, Microsoft, and Google.