We bought a house 7 months ago and since 4 months the toilet would overflow if we use the wash machine, the dish washer or the sink in the kitchen or bathroom. I would see the lint from the washer and I would see the food from our garbage disposal. Also to mention is that it is an old house from the late 70's and it is galvanized pipes. Is it smart to clean them or easier to replace them with plastic pipes?
And if to clean, how?

All of those fixtures sit higher up than the toilet. If your main drain clogs the backup could cause the overflow. However, your shower or bath tub should back up quicker, does it?

We don't have a shower or bathtub there. It is just the Guest Bath in our Laundry room. It takes a while to back up in the toilet. And sometimes the water stands in the dish washer.

Do you have a tub or shower eslewhere on this same floor? This will help determine where the clog is located. Is level on a concrete slab or wood structure, any crawl or basement under it?

It is just a 1-Story house but the other bath rooms are on the other side of the house and working fine. Never have problems with them. There is no crawl space or basement and I think it is on concrete slab.

You will need to do some seaching but on a slab there is a clean out with a cap or plug. Remove the plug and rod the line, do not add any chemicals. The clean out for my home is actually on the wall with a decorative covering.

Thanks, I will take a look.

If you don't find the clean out you might try calling the local building department who frequently keeps blueprints for decades