Personally I never understood why people diversify when it comes to stocks and not so much when it comes to bonds. I know a few people that just buy bonds of 2-3 companies - that sounds too risky to me!
I started investing in bonds a few years back when reading on WSJ on so called maturity bond ETFs. ishares offers those - another company as well - dont remember the name. Depending on your risk appetite you can choose between treasuries, investment grade, muni and high yield="junk".
A maturity bond ETF (sometimes other terms used) is an ETF that buys bonds with similar duration and has a fixed end date. This is opposed to normal - and much more common - bond ETF that does regular "roll-over" and replaces the bonds so that the ETF doesn't end.
The advantage of buying maturity ETFs is that you can make a so called bond ladder and do some strategic choices. For instance, after yields were dropping in 2022 it looked quite attractive to me to lock in a 5-9% yield in USD for up to 10 years, with an additional chance to sell earlier with a capital gain. Or - assuming you have a date where you will need that money again - you can chose a duration that matches this without exposing yourself to the interest volatility. That's the point of a normal bond - yet with the normal bond ETF that fixed maturity date doesn't work due to the constant rollover.
Another important aspect is the currency. You will find most offers in USD. And indeed I don't see much point to invest in bonds in CHF given the low interest rates. So it only makes sense if you want/can take the USD, EUR, or eg. GBP risk.