how to clean up i mac 2019 using clean my max x before installing Big Sur

Do not install CleanMyMac. It's considered "junkware" and is definitely not safe.

I have Big Sur installed on five of my Macs, so yes, I would consider it "worth doing".

If you feel like cleaning out some caches, not that you need to, boot into Safe Mode then restart.

How to use safe mode on your Mac - Apple Support

I delete my browser history/cache on a regular basis - since today's machines are so fast, there really is no need to keep this in the browser "memory"; the websites launch fast enough without that.

The only time you need to clean a cache is if it is corrupted. Doing it otherwise is simply slowing your machine for exactly zero benefit. Cleaning Caches is the 'Repair Permissions' voodoo these days.

I'm happy to disagree. I guess I don't understand what a bloated browser is. But you do and see a benefit from it, then by all means do it. My point is that one of the "features" of so many "cleaning" apps is cache removal for all sorts of OS functions and applications, and, as I say, this achieves exactly nothing. Disk space that is "recovered" is soon repopulated with the rebuilt caches and so on. Apart from the - vanishingly rare - event of a corrupted cache, the action is voodoo, much like repairing permissions used to be.

You're welcome. There's no need to "clean up" beyond deleting any files you don't need or want. I've owned Macs for 3+ decades and have never cleaned up any of them.

Please note that I am only referring to browser history and cache. We’ll need to disagree; there is no longer any slowing down unless you have an old Mac with an old OS, slow processor, and a lack of RAM. The browser just gets bloated.