One great thing about audio equipment is that, in utter contrast to most other modern electronics, it can last a very long time. Receivers are no exception – there are many great products that are 10-20+ years old and still hold their own. In fact, many would claim some of the best receivers still were made many years ago.
Source technology has completely changed in the last 20 years though. Sure, there are vintage hobbyists who still use CDs and vinyl and such, but most people are streaming music wirelessly via phones, laptops, etc. Bluetooth is the predominant wireless technology used with audio equipment now. Most modern receivers have onboard Bluetooth and lack any kind of physical media input.
So what if you want to stream music from your phone but you have one of those beastly receivers made in the 90s before Bluetooth existed? Can you add Bluetooth compatibility to it? Yes! And quite easily and cheaply. All you need is a Bluetooth adapter like so:

The most important thing to make sure is that the adapter has RCA inputs (the recognizable red and white slots you can see above) because any receiver, no matter how old, will have RCA line-outs – RCA has been around since the 1930s and is still ubiquitous today. The product above and many others also have an aux input which will be compatible with any standard 3.5mm plug and is a viable alternative to RCA.
All you need to do to control your old-school receiver via Bluetooth is hook up the adapter with an RCA cable:

…then pair your device to the adapter and you’re good to go.
There is one limitation with using a Bluetooth adapter over a modern receiver with onboard Bluetooth: you won’t be able to adjust the amplifier-volume (presumably the big volume knob on the receiver itself), only the source-volume. One might reasonably think you can just turn the receiver’s volume lower and then use the range of the source-volume to adjust the loudness to comfort, and this might work fine, but it’s not ideal. Reason being it’s generally best practice to put your digital source volume at or close to 100% and adjust the amplifier volume to comfort because that best preserves the audio signal along the chain.
Further reading: should I turn up my digital source volume or speaker/receiver/amplifier volume?
If you don’t mind getting up to adjust the volume on your receiver then no problem, but that does admittedly kind of defeat the point of adding Bluetooth to begin with. If you want to use your source to adjust the volume, try and do so by keeping it in the 70%-100% range as that should ensure the digital signal is adequately preserved before it gets to your receiver.