What's the best way to actually "type" special UTF-8 chars?

Everything on my system (that needs it) supports UTF-8 just fine.
That's all nice when you want output... But what if you want easy **in**put ? At the moment the only non-ASCII chars I can easily type are chars like é by using AtlGr.
But for chars like ₂ ² ≈ √ π 😀 at the moment I have to: 1. Open a browser 2. Surf to https://www.utf8icons.com or a similar site 3. Click, type and search a lot on the site to get to a page that contains the symbol i want 4. Copy it 5. Paste it in the program where I need it 6. (Optionally) close the browser What I'm looking for is a program that can do something like this: - Run in the background in a modern desktop environment (in my case Cinnamon) - Jump to the foreground to show a whole list of reasonably popular UTF-8 symbols after pressing something like F1 - Let me click a symbol after which it will be sent to the program I was last using as if it was a keypress - Give me the option to configure it to either stay visible after this "fake keypress" or jump back to background In short: Are there virtual keyboard programs with support for non-ASCII UTF-8 ? Actually... I am already happy with *any* method that improves mine. **Edit:** *For others ending up here and don't want to read all the answers themselves (or add a answer that's already given):
These are the options already mentioned + links to the answers + pro's and contra's.
Feel free to add extra solutions below (after providing them as detailed answer)*: - ibus (usually with CtrlShiftE) → Can't get it to work on Cinnamon - onboard → *pro*: Seems to do everything I need + has support for snippets, *con*: Only (by default) included non-latin layout is for math, other layouts with popular UTF-8 chars have to be created manually - gucharmap → *pro:* Lots of chars and easy to search *con:* Doesn't easily jump between foreground/background (can probably be handled with a workaround in Cinnamon itself) - kcharselect→ Same pro/con as gucharmap - Solutions from the programs themselves (e.g. Ctrl. for a couple of them) → *pro*: Ideal for that exact program *con*: Most programs, including the ones where it's needed the most, don't have one + it's not uniform - https://www.unicodeit.net/ → *pro*: Good for long math formula's. *con*: Same problem as the one I originally stated + useless for non-math symbols - Keyboard with extra symbols → *pro*: Easy *con*: Small amount of chars + extra keyboard needed for each system - Shortcuts for the most used chars with xcompose → *pro*: Easy *con*: Depending on your memory (as human, not as computer) it only works for a limited amount of chars - HTML entities to compose - *pro/con*: Too much of each, see answer - Use CtrlShiftU, Hexcode,Space : *pro/con*: Same as above