Short precise questions make one tick. I did my best to collect answers in the same manner.
ACS is algorithm for channel selection ?
Yes.
Which is better ACS or CCA ?
Those can not be compared. Supposed the acronyms mean what they are written to above, ie. Automatic Channel Selection (ACS) and Clear Channel Assessment (CCA).
ACS explained here: https://wireless.wiki.kernel.org/en/users/documentation/acs
CCA explained here (one source from many, the definitive being IEEE 802.11-2016 Standard): https://www.extremenetworks.com/extreme-networks-blog/what-is-a-clear-channel-assessment-cca/
Dynamic Frequency Selection (DFS) in channels on 5GHz is one more acronym affecting channel selection, description 1: https://forum.openwrt.org/t/wireless-config-channel-auto/1015/2 and description 2 https://en.wikipedia.org/wiki/Dynamic_frequency_selection.
A note: CCA and DFS are defined in 802.11 ammendments and standards, ACS seems not.
Disclaimer: I'm not an expert in 802.11 radios or standards, just a nerd trying to understand the pieces the 802.11 is made of and who's controlling what.