Bitcoin Systemic Changes

[Just Developer]

Does the Bitcoin system still work with the design declared as manifesto? Have any design or structural changes been made on the system?
Who decides the changes to be made on the system? According to what criteria do they make their decisions?

[Just Developer]

For example, BTC developers decided to move to a better digital signature algorithm. https://bitcoincore.org/en/2017/03/23/schnorr-signature-aggregation/

This decision is positive for BTC. Does the developers have the authority to make such changes on the system? Do they get the idea of BTC users when making these kinds of decisions?

[unamis76]

Design declared as manifesto? If you mean with what was described in the whitepaper, yes. No major changes have been made... Otherwise it probably wouldn't be Bitcoin anymore. Changes are decided by everyone and criteria for change is need for change... (Or just general ways to improve Bitcoin).

Changes are proposed in BIP's (Bitcoin Improvement Proposal), you might want to read about that.

Regarding your second post, developers can make clients and changes to clients and can author them as they wish (if people use those clients or not is a different talk).

[AgentofCoin]

Does the Bitcoin system still work with the design declared as manifesto?

No. Some things have naturally fallen away due to technology and experience.
In its overall design theory, it still functions well.

Have any design or structural changes been made on the system?

Not major changes, that I am aware of.
The whitepaper is a very simplistic outline and many things are
different now compared to 2009.

Who decides the changes to be made on the system?

The Validator Node Network in conjunction with the Miner Nodes.

According to what criteria do they make their decisions?

If a proposal receives a majority of approval signaling, then at a certain point in the
future whether manually or automatically, majority agrees to switch to that new change.
That is the current Consensus mechanism for "changes".

Does the developers have the authority to make such changes on the system?

No, Developers only propose different ideas and possibilities.
The Validator Node Network and Miner Nodes make the changes.
If the nodes don't like a proposal, they do not support it and thus no change occurs.

Do they get the idea of BTC users when making these kinds of decisions?

Developers do not make decisions, they propose different ideas and possibilities.
Decisions are made by the Validator Node Network and Miner Nodes.
The change only comes about due to their Consensus.