Well, too many words and no concrete recommendations, as usual.
I hate to break it to you, but this is 'real' RF engineering and that's simply not something that you can condense into a 150 character Twitter post or some dumbed down tiktok video.
Thanks again. I will try to understand things you said here But here is some more specific information about my devices:
My computer doesn't have an HDMI port, so I use a Mini DP to HDMI adapter. Then, I have a cable with HDMI on one end and then DVI on the monitor.
And the speakers have an external amplifier that seems to be homemade, at least partially. The cord in the right speaker was extended so it can reach the amplifier that is on the left side of my table.
Hope these would help with troubleshooting.
The source of the problem is likely non-existent or failed mains filter caps in your lamps and fridge. They are not supposed to interfere that badly. I have a room full of vintage tech, and most of my fluorescents don't interfere on any of them - except for one AC/DC radio.
You can put snubber networks over the switch contacts of your lamps, and over the compressor relay contacts of your fridge:
https://uk.farnell.com/roxburgh/re120033/cap-0-033-f-275vac-20/dp/2336107If you cannot install them yourself, ask your local repair place or electrician. Usually it takes at most 10 minutes to install one, though it depends on how much space there actually is available.
I chose one by gut feel so no guarantees but here's a bit of info about how to 'properly' select them:
https://www.digikey.com/en/articles/resistor-capacitor-rc-snubber-design-for-power-switches edit: disregard, that's about a whole different style of switch.
With regards to the speakers and amp - remove the input cable from the amp. Solved? Plug it back in, but unplug the signal source end. Solved? Likely a ground loop issue. Not solved? Bad shielding, or interference entering via the speaker or mains cable.
Still issues? Remove the long speaker cable. If it gets less, then your interference enters via the long section of unshielded speaker cable. You can suppress interference entering via that pathway by looping the wire through a ferrite core like this one:
https://nl.aliexpress.com/item/1005004130655236.htmlI typically use ferrite or iron powder cores scavenged from PC power supplies, it's not really important which one it is. Today it's not a common pathway for interference anymore, but in the CB radio era it happened quite regularly that interference entered via the speaker output.
No influence? Then add a mains filter like the one i posted before to your amplifier. Sometimes you can find them in power strip form for 'audiophile' use. It won't improve sound quality but it can reduce RF interference. But cheaper is to just use a filter module to build into the amp, if there's enough space.
With regards to the HDMI - ferrite clamps around both ends of the cable may work, but don't expect them to do wonders. They function only on the 'top' part of the RF spectrum, leaving the 'bottom' part unaffected. So if the interference is at the bottom, they won't help.
Better shielded cable might work, but again no guarantees, if the problem is the lamps that are unusually high interference.